Remove unused Dictionary defaultable subscript

Author: cysp

Sources/Apollo/Collections.swift

@@ -1,17 +1,3 @@
-extension Dictionary {
-  subscript(key: Key, withDefault value: @autoclosure () -> Value) -> Value {
-    mutating get {
-      if self[key] == nil {
-        self[key] = value()
-      }
-      return self[key]!
-    }
-    set {
-      self[key] = newValue
-    }
-  }
-}
-
 public extension Dictionary {
   static func += (lhs: inout Dictionary, rhs: Dictionary) {
     #if swift(>=3.2)