Remove obsolete GLES3 backend

Due to the port to Vulkan and complete redesign of the rendering backend,
the `drivers/gles3` code is no longer usable in this state and is not
planned to be ported to the new architecture.

The GLES2 backend is kept (while still disabled and non-working) as it
will eventually be ported to serve as the low-end renderer for Godot 4.0.

Some GLES3 features might be selectively ported to the updated GLES2
backend if there's a need for them, and extensions we can use for that.

So long, OpenGL driver bugs!

drivers/SCsub

@@ -24,7 +24,6 @@ SConscript('winmidi/SCsub')
 
 # Graphics drivers
 if (env["platform"] != "server"):
-#    SConscript('gles3/SCsub')
 #    SConscript('gles2/SCsub')
     SConscript('vulkan/SCsub')
     SConscript('gl_context/SCsub')