gl-renderer: Remove warning on missing extensions

Not having swap_buffers_with_damage could cause a performance impact on
some backends, but at least on GBM it causes no issues. It also seems to
confuse users into thinking it's a legitimate error which could explain
session slowness.

Similarly with buffer_age, whilst we do lose a little bit of performance
by not being able to do partial renders, it is not a great deal, and the
user is unlikely to be able to do anything about it in any event.

Remove the warning; we print the full extension list at startup, so we
already have enough information from the logs to easily diagnose any
real errors.

Signed-off-by: Daniel Stone <daniels@collabora.com>