bisect--helper: retire `--bisect-autostart` subcommand

bisect--helper: retire `--bisect-autostart` subcommand

The `--bisect-autostart` subcommand is no longer used from the
git-bisect.sh shell script. Instead the function
`bisect_autostart()` is directly called from the C implementation.

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>