mesa: Drop manual checks for outside begin/end.

mesa: Drop manual checks for outside begin/end.

We now have a separate dispatch table for begin/end that prevent these
functions from being entered during that time.  The
ASSERT_OUTSIDE_BEGIN_END_WITH_RETVALs are left because I don't want to
change any return values or introduce new error-only stubs at this
point.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>