t1450: refactor loose-object removal

t1450: refactor loose-object removal

Commit 90cf590f5 (fsck: optionally show more helpful info
for broken links, 2016-07-17) added a remove_loose_object()
helper, but we already had a remove_object() helper that did
the same thing. Let's combine these into one.

The implementations had a few subtle differences, so I've
tried to take the best of both:

  - the original used "sed", but the newer version avoids
    spawning an extra process

  - the original processed "$*", which was nonsense, as it
    assumed only a single sha1. Use "$1" to make that more
    clear.

  - the newer version ran an extra rev-parse, but it was not
    necessary; it's sole caller already converted the
    argument into a raw sha1

  - the original used "rm -f", whereas the new one uses
    "rm". The latter is better because it may notice a bug
    or other unexpected failure in the test. (The original
    does check that the object exists before we remove it,
    which is good, but that's a subset of the possible
    unexpected conditions).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>