Follow-up Ib933de3a3f: remove unnecessary override from SpecialWatchlist

Follow-up Ib933de3a3f: remove unnecessary override from SpecialWatchlist

isStructuredFilterUiEnabledByDefault() should no longer be overridden,
overrides should be put in the static checkStructuredFilterUiEnabled()
instead. That function was already added, but the duplicated non-static
function wasn't removed.

It looks to me that ?rcfilters=1 shouldn't have been working on
Watchlist for this reason, but it was working, and I can't figure out
why.

Change-Id: Iaf962b4e0428e262b57dc12ee0ba2cb6f854eb7e