Delete peekier.xml search provider and fix behavior when a search engine is removed
This is resubmit of !563 (closed) with additions on the fix of when a search engine is removed. The original description follows:
Peekier has been decommissioned (superseded by Kagi, but we won't ship that as a selectable browser search engine, by default, due to its non-free nature).
Edited by Ratchanan Srirattanamet