dri/swrast: make unbind a noop

This is for loading swrast_dri.so from libGL.

MakeContextCurrent() seems to unbind the context right after binding it and
DRI drivers also have a noop DriverAPI.UnbindContext ...

src/mesa/drivers/dri/swrast/swrast.c

@@ -695,7 +695,6 @@ static int driUnbindContext(__DRIcontext *ctx)
 {
     TRACE;
     (void) ctx;
-    _mesa_make_current(NULL, NULL, NULL);
     return GL_TRUE;
 }