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!

platform/android/java/app/AndroidManifest.xml

@@ -15,7 +15,6 @@
         android:largeScreens="true"
         android:xlargeScreens="true" />
 
-    <!-- glEsVersion is modified by the exporter, changing this value here has no effect. -->
     <uses-feature
         android:glEsVersion="0x00020000"
         android:required="true" />