glsl: remove element_type() helper

glsl: remove element_type() helper

We now have is_array() and without_array() that make the
code much clearer and remove the need for this.

For all remaining calls to this we already knew that
the type was an array so returning a null wasn't adding any value.

v2: use without_array() in _mesa_ast_array_index_to_hir() and don't use
 without_array() in lower_clip_distance_visitor() as we want to make sure the
 array is 2D.

Reviewed-by: Matt Turner <mattst88@gmail.com>