Remove Support for Old Setting Names

[wesleyboar]

Complaint

Too much code for old setting names that are not used anymore.

Proposal

Remove the code for old setting names.

Alternative

Ignore this unused code.

Proof

In TACC/Core-Portal-Deployments@30a41f6, the old setting names are not used:

SEARCH_	INCLUDES	FAVICON	TACC_
replaced by PORTAL_SEARCH_	replaced by PORTAL_IS_/PORTAL_HAS_	replaced by PORTAL_FAVICON	current settings' names

[wesleyboar]

Technically, removing backwards-compatibility is a breaking change (i.e. an incompatible API change), so I suggest doing this only for v5 release. The v5 release would likely come when Core-CMS uses Django v5.