mediawiki.action.watch.ajax: Don't get wgPageName

Follows-up r113737 / 72c33b452369. Client-side page caching
for logged-in users should no longer be a concern by now :)

Change-Id: Ifeefa22fb2e0fc05833bf89123d744314429fe36
Merge "mediawiki.action.watch.ajax: Don't get wgPageName"