st/dri: Remove useless flush front.

st/dri: Remove useless flush front.

In the following scenario:
- CreateContext C1
- MakeCurrent C1
- DestroyContext C1 (does not actually destroy the first context, postponed
 until the next MakeCurrent)
 - CreateContext C2
 - MakeCurrent C2
MakeCurrent will call flush on a half destroyed context, leading to crashes.
Since the other paths (destroy and makecurrent) already flush the context,
there is no need to flush here, so we remove this useless flush front call.

This fixes GPU crashes with Chrome and gallium drivers.