test-lib: remove unused $_x40 and $_z40 variables

test-lib: remove unused $_x40 and $_z40 variables

These two have fallen out of use with the SHA-256 migration.

The last use of $_x40 was removed in fc7e73d7ef (t4013: improve
diff-post-processor logic, 2020-08-21) and

The last use of $_z40 was removed in 7a868c51c2 (t5562: use $ZERO_OID,
2019-12-21), but it was then needlessly refactored to be hash-agnostic
in 192b517589 (t: use hash-specific lookup tables to define test
constants, 2020-02-22). We can just remove it.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>