rebase -i: stop setting GIT_CHERRY_PICK_HELP

rebase -i: stop setting GIT_CHERRY_PICK_HELP

Setting this environment variable causes the sequencer to display a
custom message when it stops for the user to resolve conflicts and
remove CHERRY_PICK_HEAD. Setting it in "git rebase" is a vestige of
the scripted implementation, now that it is a builtin command we do
not need to communicate with the sequencer machinery via environment
variables.

Move the conflicts advice to use when rebasing into
sequencer.c so we do not need to pass it via the environment.

Note that we retain the changes in e4301f73fff (sequencer: unset
GIT_CHERRY_PICK_HELP for 'exec' commands, 2024-02-02) just in case
GIT_CHERRY_PICK_HELP is set in the environment when "git rebase" is
run.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>