mesa: remove an optional GL error about mapped buffers during execution

mesa: remove an optional GL error about mapped buffers during execution

Not having this here, even if the branch is not taken, increases
CPU performance by 2% on radeonsi. If some drivers need this, the spec
does allow GL termination, meaning abort(), which is a more effective
alternative given that this never happens.

You may ask, do we really pay a 2% performance hit for every conditional
not taken? For some of them, we do.

Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8798>