There are no callers left of write_file_gently(). Let's drop
it, as it doesn't seem likely for new callers to be added
(since its inception, the only callers who wanted the gentle
form generally just died immediately themselves, and have
since been converted).

While we're there, let's also drop the "int" return from
write_file, as it is never meaningful (in the non-gentle
form, we always either die or return 0).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>