datetime: remove unused function datetime_gmtoff

The function `datetime_gmtoff()` has been introduced in commit
aec6fbac ("datetime: implement date.isdst") for a function
`datetime_increment_by()`. In commit 6ca07285
("datetime: fix interval arithmetic for DST") the function
`datetime_increment_by()` has been updated and call of the
function `datetime_gmtoff()` was removed. `datetime_gmtoff` is
dead code now. The patch removes `datetime_gmtoff()`.

NO_CHANGELOG=codehealth
NO_DOC=codehealth
NO_TEST=codehealth

(cherry picked from commit 625afe418151de1015483ed24acaa57e1f85efb3)