pull: remove support for `--rebase=preserve`

pull: remove support for `--rebase=preserve`

In preparation for `git-rebase--preserve-merges.sh` entering its after
life, we remove this (deprecated) option that would still rely on it.

To help users transition who still did not receive the memo about the
deprecation, we offer a helpful error message instead of throwing our
hands in the air and saying that we don't know that option, never heard
of it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>