Drop checks with last alternate superblock in fsck_ffs(8).

The fsck_ffs(8) utility made sanity checks of critical superblock
fields by comparing the values of those fields in the standard
superblock againt the values of those fields in the last alternate
superblock. The code for validating a superblock now cover all the
checked fields as well as many more. Further the checks done are
far more comprehensive. So we now drop the alternate superblock
checks as they no longer provide value. Dropping these checks also
eliminates the need to read the alternate superblock.