SiteStats::jobs fix when there is a single job

SiteStats::jobs fix when there is a single job

wfSpecialStatistics() used a database to store the jobs. When estimating
the number of row in the `job` table, even when there is no row MySQL
would still do a single row read. Hence it erroneously returned 1.

That got fixed in 7cd0b2e0b (T12228) with a hack.

Nowadays, the functionality has been moved to SiteStats which is backed
up by a JobQueueGroup.  It now returns the exact number of jobs and the
hack is no more needed.  We failed to remove it when porting it to the
JobQueue system.

Change-Id: I6ef55b3dcff3e47a92e2eeeddcbae078654f815a