ffs: retire unused fsckpid mount option

The fsckpid mount option was introduced in 927a12ae16433b50 along with a
couple sysctl's to support SU+J with snapshots. However, those sysctl's
were never used and eventually removed in f2620e9ceb3ede02.

There are no in-tree consumers of this mount option.

Reviewed by: mckusick, kib
Differential Revision: https://reviews.freebsd.org/D32015