t1405: remove unneeded cleanup step

t1405: remove unneeded cleanup step

In 5e00514745 (t1405: explictly delete reflogs for reftable, 2022-01-31)
we have added a test that explicitly deletes the reflog when not using
the "files" backend. This was required because back then, the "reftable"
backend didn't yet delete reflogs when deleting their corresponding
branches, and thus subsequent tests would fail because some unexpected
reflogs still exist.

The "reftable" backend was eventually changed though so that it behaves
the same as the "files" backend and deletes reflogs when deleting refs.
This was done to make the "reftable" backend behave like the "files"
backend as closely as possible so that it can act as a drop-in
replacement.

The cleanup-style test is thus not required anymore. Remove it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>