Remove long-dead OutputPage methods set/getPageTitleActionText()

Remove long-dead OutputPage methods set/getPageTitleActionText()

They were accidentally made non-functional in April 2009 by commit
e4d21170.  Subsequent commits 2d045fa1, 9e230f30, e275ea28, ae45908c
removed all callers by October 2011.  Needless to say, I found no
callers in core or extensions.

It seems we have the functionality in some other way, probably just by
directly calling setHTMLTitle(), so there's no need to revive this
feature.

Bug: T200643
Change-Id: Ifa2537b97cf19e7e91ee33d4ea4c131f7a38ee89