bisect--helper: retire `--check-expected-revs` subcommand

bisect--helper: retire `--check-expected-revs` subcommand

The `--check-expected-revs` subcommand is no longer used from the
git-bisect.sh shell script. Functions `check_expected_revs` and
`is_expected_revs` are also deleted.

Mentored-by: Lars Schneider <redacted>
Mentored-by: Christian Couder <redacted>
Mentored-by: Johannes Schindelin <redacted>
Signed-off-by: Pranit Bauva <redacted>
Signed-off-by: Tanushree Tumane <redacted>
Signed-off-by: Miriam Rubio <redacted>
Reviewed-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>