Remove non-working help URLs from all Wikibase API modules

The actual API documentation (the one you see when calling api.php
with no paramaters) is the best we currently have. It should not
link to outdated or even missing wiki pages. :-(

In case of questions, ask Lydia, please. ;-)

Change-Id: Ibf8759841fd374c465da8fdf901ae25e67e9cfad
Merge "Remove non-working help URLs from all Wikibase API modules"