commit-graph: remove unnecessary UNLEAK

commit-graph: remove unnecessary UNLEAK

When f4dbdfc4d5 (commit-graph: clean up leaked memory during write,
2018-10-03) added the UNLEAK, it was right before a call to die_errno().
e103f7276f (commit-graph: return with errors during write, 2019-06-12)
made it unnecessary, as it was then followed by a free() call for the
allocated string.

The code moved to write_commit_graph_file() in the meantime and the
string pointer is now part of a struct, but the function's only caller
still cleans up the allocation.  Drop the superfluous UNLEAK.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>