Removes playgrounds from apollo-ios (#1905)

* Remove ApolloMacPlayground
* Remove ApolloCodegenFrontendPlayground
* Delete Apollo Playground.xcscheme
* Update RELEASE_CHECKLIST.md
* Update README.md linking to the playground repo
* Update docs with link to new playground repo

Author: calvincestari

Apollo.xcodeproj/project.pbxproj

@@ -626,7 +626,6 @@
 		9BAEEC14234C132600808306 /* CLIExtractorTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CLIExtractorTests.swift; sourceTree = "<group>"; };
 		9BAEEC16234C275600808306 /* ApolloSchemaTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ApolloSchemaTests.swift; sourceTree = "<group>"; };
 		9BAEEC18234C297800808306 /* ApolloCodegenTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ApolloCodegenTests.swift; sourceTree = "<group>"; };
-		9BB1DAC624A66B2500396235 /* ApolloMacPlayground.playground */ = {isa = PBXFileReference; lastKnownFileType = file.playground; name = ApolloMacPlayground.playground; path = Playgrounds/ApolloMacPlayground.playground; sourceTree = SOURCE_ROOT; xcLanguageSpecificationIdentifier = xcode.lang.swift; };
 		9BB4F5B12581AA50004F0BD6 /* CacheDependentInterceptorTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CacheDependentInterceptorTests.swift; sourceTree = "<group>"; };
 		9BC139A224EDCA4400876D29 /* InterceptorTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = InterceptorTests.swift; sourceTree = "<group>"; };
 		9BC139A524EDCAD900876D29 /* BlindRetryingTestInterceptor.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = BlindRetryingTestInterceptor.swift; sourceTree = "<group>"; };
@@ -671,7 +670,6 @@
 		9F3910262549741400AF54A6 /* MockGraphQLServer.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = MockGraphQLServer.swift; sourceTree = "<group>"; };
 		9F41CBEF25A3490600C02CB7 /* schema.graphqls */ = {isa = PBXFileReference; lastKnownFileType = text; path = schema.graphqls; sourceTree = "<group>"; };
 		9F438D0B1E6C494C007BDC1A /* BatchedLoadTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = BatchedLoadTests.swift; sourceTree = "<group>"; };
-		9F454784259390B9001783F6 /* ApolloCodegenFrontendPlayground.playground */ = {isa = PBXFileReference; lastKnownFileType = file.playground; name = ApolloCodegenFrontendPlayground.playground; path = ../../../Playgrounds/ApolloCodegenFrontendPlayground.playground; sourceTree = "<group>"; xcLanguageSpecificationIdentifier = xcode.lang.swift; };
 		9F54C8B4255D760B0065AFD6 /* ApolloPerformanceTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = ApolloPerformanceTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
 		9F54C8B6255D760B0065AFD6 /* ParsingPerformanceTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ParsingPerformanceTests.swift; sourceTree = "<group>"; };
 		9F54C8B8255D760B0065AFD6 /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
@@ -1435,7 +1433,6 @@
 		9FC750461D2A532C00458D91 /* Apollo */ = {
 			isa = PBXGroup;
 			children = (
-				9BB1DAC624A66B2500396235 /* ApolloMacPlayground.playground */,
 				9FC750621D2A59F600458D91 /* ApolloClient.swift */,
 				9B708AAC2305884500604A11 /* ApolloClientProtocol.swift */,
 				9FC750601D2A59C300458D91 /* GraphQLOperation.swift */,
@@ -1560,7 +1557,6 @@
 		9FE1E54C2588C5E000AA967E /* Frontend */ = {
 			isa = PBXGroup;
 			children = (
-				9F454784259390B9001783F6 /* ApolloCodegenFrontendPlayground.playground */,
 				9F1A9665258F34BB00A06EEB /* ApolloCodegenFrontend.swift */,
 				9F62DFCF2590710E00E6E808 /* GraphQLSource.swift */,
 				9F1A9667258F34BB00A06EEB /* GraphQLSchema.swift */,