Remove blur helper functions (AcademySoftwareFoundation#2498)

Following up on AcademySoftwareFoundation#2411, this changelist removes the blur helper functions in GLSL, MSL, and OSL that are no longer used by the codebase.

Author: jstone-lucasfilm

libraries/stdlib/genglsl/lib/mx_sampling.glsl

@@ -31,7 +31,6 @@ MATERIALX_NAMESPACE_BEGIN
 
 const string GlslShaderGenerator::TARGET = "genglsl";
 const string GlslShaderGenerator::VERSION = "400";
-const string GlslSamplingIncludeFilename = "stdlib/genglsl/lib/mx_sampling.glsl";
 
 //
 // GlslShaderGenerator methods

libraries/stdlib/genmsl/lib/mx_sampling.metal

@@ -35,7 +35,6 @@ MATERIALX_NAMESPACE_BEGIN
 
 const string MslShaderGenerator::TARGET = "genmsl";
 const string MslShaderGenerator::VERSION = "2.3";
-const string MslSamplingIncludeFilename = "stdlib/genmsl/lib/mx_sampling.metal";
 
 //
 // MslShaderGenerator methods