skins: Deprecate MediaWikiI18N::translate()

skins: Deprecate MediaWikiI18N::translate()

Remove all usage of $tpl->translator->translate() with wfMessage()->text(),
which does the same thing, given the deprecated $tpl->translator->set()
and $tpl->setTranslator() methods have no callers.

These new wfMessage() callers should probably be changed to context calls
via BaseTemplate::msg() or Skin::msg() at some point, but I'm reserving that
for a separate commit given wfMessage() is what MediaWikiI18N::translate()
currently does - to reduce risk of unrelated behaviour changes.

While `$tpl->setTranslator` and `MediaWikiI18N::set` ($tpl->translator->set)
have no known callers in Wikimedia Git, the `$tpl->translator->translate`
method does have a handful of calls (although less than three, and not in
bundled or WMF-deployed repos). As such, proceeding with hard-deprecation.

Bug: T186090
Change-Id: I93d503de5515298288852ec4c150959fd688786b