FreeBSD: taskq: Remove unused declaration

Variable 'uma_align_cache' has not been used since commit "FreeBSD: Use
a hash table for taskqid lookups" (3933305ea). Moreover, it is soon
going to become private to FreeBSD's UMA in 15.0-CURRENT (main),
14.0-STABLE (stable/14) and 13.2-STABLE (stable/13). Should accessing
this information become necessary again, one will have to use the new
accessors for recent versions.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olivier Certner <olce.freebsd@certner.fr>
Closes #15416