glsl: Remove GLSL_TYPE_FUNCTION define.

Functions are not first class objects in GLSL, so there is never a value
of function type.  No code actually used this except for one function
which asserted it shouldn't occur.  One comment mentioned it, but was
incorrect.  So we may as well remove it entirely.

src/glsl/ir_function.cpp

@@ -70,7 +70,6 @@ type_compare(const glsl_type *a, const glsl_type *b)
        */
       return (type_compare(a->fields.array, b->fields.array) == 0) ? 0 : -1;
 
-   case GLSL_TYPE_FUNCTION:
    case GLSL_TYPE_VOID:
    case GLSL_TYPE_ERROR:
    default: