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/docs/06_functions_list.dox b/docs/06_functions_list.dox
index 19485a8..1f4794f 100644
--- a/docs/06_functions_list.dox
+++ b/docs/06_functions_list.dox
@@ -284,36 +284,7 @@
         - @ref CLWarpPerspective
         - @ref CLWinogradInputTransform
 
-@section S6_3 GLES Compute functions
-
-- @ref IFunction
-    - @ref GCBatchNormalizationLayer
-    - @ref GCConcatenateLayer
-    - @ref GCConvolutionLayer
-    - @ref GCConvolutionLayerReshapeWeights
-    - @ref GCDepthwiseConvolutionLayer3x3
-    - @ref GCDirectConvolutionLayer
-    - @ref GCDropoutLayer
-    - @ref GCFullyConnectedLayer
-    - @ref GCGEMM
-    - @ref GCNormalizationLayer
-    - @ref GCNormalizePlanarYUVLayer
-    - @ref GCPoolingLayer
-    - @ref GCSoftmaxLayer
-    - @ref IGCSimpleFunction
-        - @ref GCAbsoluteDifference
-        - @ref GCActivationLayer
-        - @ref GCArithmeticAddition
-        - @ref GCFillBorder
-        - @ref GCFullyConnectedLayerReshapeWeights
-        - @ref GCGEMMInterleave4x4
-        - @ref GCGEMMTranspose1xW
-        - @ref GCPixelWiseMultiplication
-        - @ref GCScale
-        - @ref GCTensorShift
-        - @ref GCTranspose
-
-@section S6_4 CPP functions
+@section S6_3 CPP functions
 
  - @ref IFunction
     - @ref CPPDetectionOutputLayer