t5503: simplify setup of test which exercises failure of backfill

In the testcase to exercise backfilling of tags for fetches we evoke a failure of the backfilling mechanism by creating a reference that later on causes a D/F conflict. Because the assumption was that git-fetch(1) would notice the D/F conflict early on this conflicting reference was created via the reference-transaction hook just when we were about to write the backfilled tag. As it turns out though this is not the case, and the fetch fails in the same way when we create the conflicting ref up front. Simplify the test setup creating the reference up front, which allows us to get rid of the hook script. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>