Clean up options with betafeature

When the betafeature is enabled CirrusSearch shouldn't be in
$wgSearchTypeAlternatives because it is not longer an alternate - it is
the default.  This was manifesting as CirrusSearch appearing twice in the
list of api options for srbackend.

Bug: 69517
Change-Id: Ic39538d4091e10b1e8c79109ba3f978bcad70d44