Remove OpenGL ES support

Remove the following:
 - Relevant backend kernels
 - Relevant backend functions
 - Relevant backend validation tests
 - Relevant backend specific examples
 - Remove backend support from Graph API
 - Remove backend support from build system

Update documentation

Resolves: COMPMID-4149

Change-Id: Id0621d6ee35169754de458103907aaba4ef770c0
Signed-off-by: Manuel Bottini <manuel.bottini@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/5097
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Reviewed-by: Georgios Pinitas <georgios.pinitas@arm.com>
diff --git a/tests/framework/ParametersLibrary.cpp b/tests/framework/ParametersLibrary.cpp
index a6953ea..5041653 100644
--- a/tests/framework/ParametersLibrary.cpp
+++ b/tests/framework/ParametersLibrary.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019 Arm Limited.
+ * Copyright (c) 2019-2021 Arm Limited.
  *
  * SPDX-License-Identifier: MIT
  *
@@ -56,12 +56,5 @@
 }
 #endif /* ARM_COMPUTE_CL */
 
-#if ARM_COMPUTE_GC
-template <>
-typename ContextType<GCTensor>::type *ParametersLibrary::get_ctx<GCTensor>()
-{
-    return static_cast<typename ContextType<GCTensor>::type *>(_gc_ctx.get());
-}
-#endif /* ARM_COMPUTE_GC */
 } // namespace test
 } // namespace arm_compute