MWTitleInputWidget: Remove prefixColon option and use relative title instead

See Ie554adef which implements this as a method on mw.Title instead.

Bug: 67386
Change-Id: I0d768fac338aed04fd2c0e4e8da8d556f94a4287
Merge "MWTitleInputWidget: Remove prefixColon option and use relative title instead"