mesa: dont set GLSLVersion in _mesa_init_constants()

Just leave it as 0 and let the drivers set it (as they already do)
to avoid redundantly initialising it.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/main/context.c

@@ -639,7 +639,6 @@ _mesa_init_constants(struct gl_constants *consts, gl_api api)
    consts->MaxGeometryTotalOutputComponents = MAX_GEOMETRY_TOTAL_OUTPUT_COMPONENTS;
 
    /* Shading language version */
-   consts->GLSLVersion = 120;
    _mesa_override_glsl_version(consts);
 
 #ifdef DEBUG