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!

misc/dist/linux/godot.6

@@ -59,7 +59,7 @@ Password for remote filesystem.
 Audio driver ('PulseAudio', 'ALSA', 'Dummy').
 .TP
 \fB\-\-video\-driver\fR <driver>
-Video driver ('GLES3', 'GLES2').
+Video driver ('Vulkan', 'GLES2').
 .SS "Display options:"
 .TP
 \fB\-f\fR, \fB\-\-fullscreen\fR

misc/dist/shell/_godot.zsh-completion

@@ -42,7 +42,7 @@ _arguments \
   '--remote-fs[use a remote filesystem]:remote filesystem address' \
   '--remote-fs-password[password for remote filesystem]:remote filesystem password' \
   '--audio-driver[set the audio driver]:audio driver name' \
-  "--video-driver[set the video driver]:video driver name:((GLES3\:'OpenGL ES 3.0 renderer' GLES2\:'OpenGL ES 2.0 renderer'))" \
+  "--video-driver[set the video driver]:video driver name:((Vulkan\:'Vulkan renderer' GLES2\:'OpenGL ES 2.0 renderer'))" \
   '(-f --fullscreen)'{-f,--fullscreen}'[request fullscreen mode]' \
   '(-m --maximized)'{-m,--maximized}'[request a maximized window]' \
   '(-w --windowed)'{-w,--windowed}'[request windowed mode]' \

misc/dist/shell/godot.bash-completion

@@ -105,7 +105,7 @@ _complete_godot_bash() {
   elif [[ $prev == "--video-driver" ]]; then
     local IFS=$' \n\t'
     # shellcheck disable=SC2207
-    COMPREPLY=($(compgen -W "GLES3 GLES2" -- "$cur"))
+    COMPREPLY=($(compgen -W "Vulkan GLES2" -- "$cur"))
   elif [[ $prev == "--path" || $prev == "--doctool" ]]; then
     local IFS=$'\n\t'
     # shellcheck disable=SC2207