glsl: Remove open coded version of ir_variable::interpolation_string().

Presumably the function didn't exist when we wrote this code.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ast_to_hir.cpp

@@ -2010,24 +2010,10 @@ apply_type_qualifier_to_variable(const struct ast_type_qualifier *qual,
    if (var->interpolation != INTERP_QUALIFIER_NONE &&
        !(state->target == vertex_shader && var->mode == ir_var_out) &&
        !(state->target == fragment_shader && var->mode == ir_var_in)) {
-      const char *qual_string = NULL;
-      switch (var->interpolation) {
-      case INTERP_QUALIFIER_FLAT:
-	 qual_string = "flat";
-	 break;
-      case INTERP_QUALIFIER_NOPERSPECTIVE:
-	 qual_string = "noperspective";
-	 break;
-      case INTERP_QUALIFIER_SMOOTH:
-	 qual_string = "smooth";
-	 break;
-      }
-
       _mesa_glsl_error(loc, state,
 		       "interpolation qualifier `%s' can only be applied to "
 		       "vertex shader outputs and fragment shader inputs.",
-		       qual_string);
-
+		       var->interpolation_string());
    }
 
    var->pixel_center_integer = qual->flags.q.pixel_center_integer;