http: cleanup function fetch_and_setup_pack_index()

http: cleanup function fetch_and_setup_pack_index()

Cleanup the function `fetch_and_setup_pack_index()` by removing the
useless call to the function `unlink()`.

This is not necessary anymore since 63aca3f7f1 (dumb-http: store
downloaded pack idx as tempfile, 2024-10-25), when `fetch_pack_index()`
started registering its return value (in this case `tmp_idx`) as a
tempfile to be deleted at process exit.

Signed-off-by: LorenzoPegorari <redacted>
Signed-off-by: Junio C Hamano <redacted>