compiler/types: Remove unused GLSL_TYPE_FUNCTION and related functions

compiler/types: Remove unused GLSL_TYPE_FUNCTION and related functions

GLSL doesn't use that type.  SPIR-V used for a while but later started
relying on its own data structures and stopped using it.
See ca62e849d3c ("nir/spirv: Stop using glsl_type for function types")

If we were ever to add this one again, would be better to have a way to
grab a key for lookup that did not require allocations, right now that's
needed to inject return type as the first element in params array.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25160>