Always link to diff in watchlist feed

Remove option to link to diffs in watchlist feed instead of the page itself, which was disabled by default.
This makes it consistent with the contributions feed when Change-Id: Ie33e2ba90e7a7566d0e0925220b8fd12b1928513 is merged.
For these changes feeds it makes sense to always link to the actual diffs.

See also bug 10268.

Change-Id: I0917844219be773ce24b4c6bac9b7d0d5a4b922d