gallium/cso: kill off non-functional shader caching

gallium/cso: kill off non-functional shader caching

Suggested by José.

We don't provide shader caching in CSO. Most of the time the api provides
object semantics for shaders anyway, and the cases where it doesn't
(eg mesa's internall-generated texenv programs), it will be up to
the state tracker to implement their own specialized caching.