mesa: (correctly) flush more in _mesa_make_current

The code being removed here requires that the old context have a
window-system drawable bound to the default framebuffer as a condition
of the context-release flush. I can find no justification for
restricting flushes to contexts like this, and GL 3.0 (etc) let you make
a context current without any drawable bound.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11423>

src/mesa/main/context.c

@@ -1607,7 +1607,6 @@ _mesa_make_current( struct gl_context *newCtx,
    }
 
    if (curCtx &&
-       (curCtx->WinSysDrawBuffer || curCtx->WinSysReadBuffer) &&
        /* make sure this context is valid for flushing */
        curCtx != newCtx &&
        curCtx->Const.ContextReleaseBehavior ==