beta: Stop using obsolete 'maxclients' param for mediawiki::web

The maxclients parameter was renamed to workers_limit in Ia424c64. We
had been improperly setting it to an undefined variable in beta
(probably due to inattentive cut-n-paste), which is the new default so
just drop the parameter all together.

Change-Id: Iffff5104b8c45ed8044ba7178954e2a6d1436ab3