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>

builtin/pack-objects.c

@@ -3603,7 +3603,6 @@ static void read_cruft_objects(void)
 			string_list_append(&discard_packs, buf.buf + 1);
 		else
 			string_list_append(&fresh_packs, buf.buf);
-		strbuf_reset(&buf);
 	}
 
 	string_list_sort(&discard_packs);