[MNT]: Remove rcParams deprecation machinery

[timhoffm]

Summary

per #15329 (comment)

We did not have any rcParams deprecations for the last 3.5 years.

Relpaces #15329.

Proposed fix

Let's get rid of the complicated code and instead

just write custom deprecation logic into setitem/getitem and add corresponding tests whenever we want to deprecate a rcparam...