t5516: drop NEEDSWORK about v2 reachability behavior

t5516: drop NEEDSWORK about v2 reachability behavior

When this test was added in 6c301adb0a (fetch: do not pass ref-prefixes
for fetch by exact SHA1, 2018-05-31), there was still some uncertainty
about the v2 protocol's looser behavior with serving objects that are
not directly pointed at by a ref.

At this point that behavior is well established, and I do not think we
would ever change v2 to match the v0 behavior (and if we did,
remembering to update this test is the least of our concerns).

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