blame: drop explicit check for commit graph

blame: drop explicit check for commit graph

Our blaming subsystem knows to use bloom filters from commit graphs to
speed up the whole computation. The setup of this happens in
`setup_blame_bloom_data()`, where we first verify that we even have a
commit graph in the first place. This check is redundant though, as we
call `get_bloom_filter_settings()` immediately afterwards which, which
already knows to return a `NULL` pointer in case we don't have a commit
graph.

Drop the redundant check.

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