Refactor out dependency on Drupal core Search

A rough first pass indicates that Funnelback module really doesn't depend on Drupal core search at all. No hooks, no config, no permissions, nothing. I'm putting together a patch for myself which I'll soon contribute back here, but we may want to add an update hook to warn module end users that it no longer needs core Search and they should disable it for themselves if they like.

I'd be happy just putting it in release notes fwiw - rather than an update hook

@larowlan Sounds fair enough. My MR is above but I expect it'll need more documentation, a changelog update, etc. Probably depends on when we want to cut a new release.