gl: drop incorrect pkg-config file for glvnd

Akin to 1a25980c ("egl: drop incorrect pkg-config file for
glvnd") and b01524ff ("meson: don't build libGLES*.so with
GLVND") , removes a pkg-config file that shouldn't have been there in
the first place, but was needed because of that GLVND bug.

Now that the glvnd bug has been fixed, it was apparent that this gl.pc
pkg-config file was forgotten to be removed, so let's do just that :)

Suggested-by: Matt Turner <mattst88@gmail.com>
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit a1de3011)