Cleanup: Remove branches that always evaluate the same way

Coverity reported that the ASSERT in taskq_create() is always true and
the *offp > MAXOFFSET_T check in zfs_file_seek() is always false.

We delete them as cleanup.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14130