Deprecate unused/unneeded pseudoconstant functions

[colemanw]

Overview

Many years ago we started restructuring our pseudoconstant classes to move away from individual functions like these. Eventually we'll remove them all.

Before

1 unused function and 2 barely used.

After

Marked deprecated for future removal.

[civibot]

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[colemanw]

@twomice can you review this one?

[eileenmcnaughton]

@colemanw I took a look at this but I struggled off the bat to very the pcp changes so I did a test & dug into the function - which gives this (bigger) alternative change #16790

[colemanw]

@eileenmcnaughton I've rebased this with your changes

[eileenmcnaughton]

@colemanw I'm OK with the change & agree the functions are not called from core -however, I think we should stick in deprecation notices rather than remove the - the alternative is more work to ensure these functions are not called in extensions

[colemanw]

Ok @eileenmcnaughton I've switched this from remove to deprecate