Remove useless $prefs array from Special:Watchlist

It is a duplicate of $defaults, so instead of calling $user->get(Bool)Option()
one more time, use the already-present value in $defaults.

I know that boolean values in $defaults are casted to integer, but
WebRequest::getBool() simply casts the result of getVal() to boolean,
so there is no problem with this.

Change-Id: I2965c8c82b8cd8bfc8dee5d81a2bfa8af9e8131f
Merge "Remove useless $prefs array from Special:Watchlist"