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/gles3/SCsub

@@ -1,7 +0,0 @@
-#!/usr/bin/env python
-
-Import('env')
-
-env.add_source_files(env.drivers_sources,"*.cpp")
-
-SConscript("shaders/SCsub")