git-bisect: remove unused SHA-1 $x40 shell variable

This variable was last used in code removed in
06f5608c14 (bisect--helper: `bisect_start` shell function partially in
C, 2019-01-02).

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

git-bisect.sh

@@ -34,8 +34,6 @@ Please use "git help bisect" to get the full man page.'
 OPTIONS_SPEC=
 . git-sh-setup
 
-_x40='[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]'
-_x40="$_x40$_x40$_x40$_x40$_x40$_x40$_x40$_x40"
 TERM_BAD=bad
 TERM_GOOD=good