grep.c: don't pass along NULL callback value

grep.c: don't pass along NULL callback value

Change grep_cmd_config() to stop passing around the always-NULL "cb"
value. When this code was added in 7e8f59d577e (grep: color patterns
in output, 2009-03-07) it was non-NULL, but when that changed in
15fabd1bbd4 (builtin/grep.c: make configuration callback more
reusable, 2012-10-09) this code was left behind.

In a subsequent change I'll start using the "cb" value, this will make
it clear which functions we call need it, and which don't.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>