configure: drop check for --enable-vaapi-glx.

configure: drop check for --enable-vaapi-glx.

This flag is obsolete. It was meant to explicitly enable/disable VA/GLX API
support, or fallback to TFP+FBO if this API is not found. Now, we check for
the VA/GLX API by default if --enable-glx is set. If this API is not found,
we now default to use TFP+FBO.

Note: TFP+FBO, i.e. using vaPutSurface() is now also a deprecated usage and
will be removed in the future. If GLX rendering is requested, then the VA/GLX
API shall be used as it covers most usages. e.g. AMD driver can't render to
an X pixmap yet.