fifolog: Trim some dead code and unused variables.

The gmt_ptr and gmt variables are not used, so the call to gmtime can
be removed entirely. In addition, there isn't a need to call
localtime twice.

Reported by: GCC -Wunused-but-set-variable
Reviewed by: phk, emaste
Differential Revision: https://reviews.freebsd.org/D40656