builtin/pack-objects.c: remove unnecessary strbuf_reset()

When reading input with the `--cruft` option, `git pack-objects` reads each line into a strbuf, and then moves it to either the list of discarded or fresh packs, depending on whether or not the input line starts with a '-' character. At the beginning of each loop iteration, the next line of input is read with `strbuf_getline()`, which calls `strbuf_reset()` (as a part of `strbuf_getwholeline()`) before reading the next line of input. Thus, the call to `strbuf_reset()` (added back in b757353676 (builtin/pack-objects.c: --cruft without expiration, 2022-05-20)) at the end of the loop is unnecessary, so let's remove it accordingly. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>