Remove obsolete GLES2 backend code

This code currently isn't compiled (and cannot compile).

We plan to re-add OpenGL ES-based renderer(s) in Godot 4.0 alongside Vulkan
(probably ES 3.0, possibly also a low-end ES 2.0), but the code will be quite
different so it's not relevant to keep this old Godot 3.2 code.

The `drivers/gles2` code from the `3.2` branch can be used as a reference for
a potential new implementation.

drivers/SCsub

@@ -24,9 +24,7 @@ SConscript("winmidi/SCsub")
 
 # Graphics drivers
 if env["platform"] != "server" and env["platform"] != "javascript":
-    #    SConscript('gles2/SCsub')
     SConscript("vulkan/SCsub")
-    SConscript("gl_context/SCsub")
 else:
     SConscript("dummy/SCsub")