t5318-commit-graph: remove unused variable

t5318-commit-graph: remove unused variable

This is a remnant from early versions of the commit-graph patch series
[1], when 'git commit-graph --write' printed the hash of the created
commit-graph file, and tests did look at the command's output, because
the commit-graph file's name included that hash as well.

[1] https://public-inbox.org/git/1517348383-112294-6-git-send-email-dstolee@microsoft.com/

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>