bisect--helper: retire `--write-terms` subcommand

bisect--helper: retire `--write-terms` subcommand

The `--write-terms` subcommand is no longer used from the
git-bisect.sh shell script. Instead the function `write_terms()`
is called from the C implementation of `set_terms()` and
`bisect_start()`.

Mentored-by: Lars Schneider <larsxschneider@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Tanushree Tumane <tanushreetumane@gmail.com>
Signed-off-by: Miriam Rubio <mirucam@gmail.com>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>