rpi-default-providers: Remove virtual/libgl provider

There are no libgl binary shipped in raspberrypi firmware repo.

This commit is based on master's branch commit:

bff90c7 ("rpi-default-providers: Remove virtual/libgl provider")
by Andrei Gherzan <andrei@gherzan.ro>.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>