rebase -i: remove unnecessary state rebase-root

rebase -i: remove unnecessary state rebase-root

Before calling 'git cherry-pick', interactive rebase currently checks
if we are rebasing from root (if --root was passed). If we are, the
'--ff' flag to 'git cherry-pick' is omitted. However, according to the
documentation for 'git cherry-pick --ff', "If the current HEAD is the
same as the parent of the cherry-picked commit, then a fast forward to
this commit will be performed.". This should never be the case when
rebasing from root, so it should not matter whether --ff is passed, so
simplify the code by removing the condition.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>