Simplify watchlist edit mode handling

The old "clear" mode has been killed years ago and now acts the same
as the "raw" mode.

Make SpecialEditWatchlist::getMode() return EDIT_RAW where it would
previously return EDIT_CLEAR, simplify handling at call sites, update
comments.

Change-Id: I1213c83b7040ae500a0299817ee503ab1b5616c6
Merge "Simplify watchlist edit mode handling"