t0410: modernize delete_object helper

The delete_object helper currently relies on a manual sed command to calculate object paths. This works, but it's a bit brittle and forces us to maintain shell logic that Git's own test suite can already handle more elegantly. Switch to 'test_oid_to_path' to let Git handle the path logic. This makes the helper hash independent, which is much cleaner than manual string manipulation. While at it, use 'local' to declare helper-specific variables and quote them to follow Git's coding style. This prevents them from leaking into global shell scope and avoids potential naming conflicts with other parts of the test suite. Helped-by: Pushkar Singh <pushkarkumarsingh1970@gmail.com> Suggested-by: Jeff King <peff@peff.net> Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>