Deprecate Parser::areSubpagesAllowed() / Parser::maybeDoSubpageLink()

Deprecate Parser::areSubpagesAllowed() / Parser::maybeDoSubpageLink()

These are unused outside the Parser and are so short it's not worth
renaming them to make them private; just hard-deprecate the methods
and inline the implementation in the small # of places they appear.

Code search:
https://codesearch.wmflabs.org/deployed/?q=areSubpagesAllowed%7CmaybeDoSubpageLink&i=nope&files=&repos=

Bug: T236810
Change-Id: Ia06c65409a3158b083bcc59c9f6e347945b375c0
(cherry picked from commit dcae22c8fa46822148dc5fd87018acb385bf4c91)