Make the GenericFunction registry fully case insensitive

Make the GenericFunction registry fully case insensitive

Registered function names based on :class:`.GenericFunction` are now
retrieved in a case-insensitive fashion in all cases, removing the
deprecation logic from 1.3 which temporarily allowed multiple
:class:`.GenericFunction` objects to exist with differing cases.   A
:class:`.GenericFunction` that replaces another on the same name whether or
not it's case sensitive emits a warning before replacing the object.

Fixes: #4649
Change-Id: I265ae19833132db07ed5b5ae40c4d24f659b1ab3