fsck_ffs: garbage collect calcsb

calcsb is unused since the logic moved to sbsearch / ffs_sbsearch in
e68866164212 ("Move the ability to search for alternate UFS superblocks
from fsck_ffs(8) into ffs_sbsearch() to allow use by other parts of the
system.")

Reviewed by: mckusick
Differential Revision: https://reviews.freebsd.org/D53038