Deprecate usage of SearchEngine:transformSearchTerm

Deprecate usage of SearchEngine:transformSearchTerm

This method was introduced in 4115586000a575eb814a80868c817d18ee169d3b
to support the prefix URI param introduced by the InputBox extension.
There are no reasons that this logic is exposed to SearchEngine users
and should be handled internally by SearchEngine implementations
that supports it.
Previously the search query was updated, now the prefix param will passed
along using SpecialSearch::$extraParams.

Bug: T198318
Change-Id: I33518d3f3ddee741ff4f3b47eb4928009bea66d1
Depends-On: I67c7f1886dd6a2d07c12015e2711c138e9f140e9