search: Remove unused deprecated methods replaced by MW Services

search: Remove unused deprecated methods replaced by MW Services

The following methods deprecated in .27 and no longer used have
been removed;

- SearchEngine::userNamespaces()
  https://codesearch.wmflabs.org/search/?q=%3A%3AuserNamespaces&i=nope&files=&repos=

- SearchEngine::namespacesAsText()
  https://codesearch.wmflabs.org/search/?q=%3A%3AnamespacesAsText&i=nope&files=&repos=

- SearchEngine::create()
  https://codesearch.wmflabs.org/search/?q=SearchEngine%3A%3Acreate&i=nope&files=&repos=

- SearchEngine::getSearchTypes()
  https://codesearch.wmflabs.org/search/?q=%3A%3AgetSearchTypes&i=nope&files=&repos=

- SearchEngine::getNearMatch()
  https://codesearch.wmflabs.org/search/?q=SearchEngine%3A%3AgetNearMatch%5C(&i=nope&files=&repos=

Also, replace deprecated use of searchableNamespaces() in SearchEngine.

Bug: T220656
Change-Id: I330fbfea158f70b2328da7c850fdb833e39b6c7d