rebase: remove -p code from git-rebase--interactive.sh

All the code specific to preserve-merges was moved to
git-rebase--preserve-merges.sh, and so it’s useless to keep it
here.

The intent of this commit is to clean this script as much as possible to
prepare a peaceful conversion as a builtin written in C.

Signed-off-by: Alban Gruin <[email protected]>

Author: agrn