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/gl_context/SCsub

@@ -1,23 +0,0 @@
-#!/usr/bin/env python
-
-Import("env")
-
-if env["platform"] in ["haiku", "osx", "windows", "linuxbsd"]:
-    # Thirdparty source files
-    thirdparty_dir = "#thirdparty/glad/"
-    thirdparty_sources = [
-        "glad.c",
-    ]
-    thirdparty_sources = [thirdparty_dir + file for file in thirdparty_sources]
-
-    env.Prepend(CPPPATH=[thirdparty_dir])
-
-    env.Append(CPPDEFINES=["GLAD_ENABLED"])
-    env.Append(CPPDEFINES=["GLES_OVER_GL"])
-
-    env_thirdparty = env.Clone()
-    env_thirdparty.disable_warnings()
-    env_thirdparty.add_source_files(env.drivers_sources, thirdparty_sources)
-
-# Godot source files
-env.add_source_files(env.drivers_sources, "*.cpp")