Revert "rebase: introduce a shortcut for --reschedule-failed-exec"

Revert "rebase: introduce a shortcut for --reschedule-failed-exec"

This patch was contributed only as a tentative "we could introduce a
convenient short option if we do not want to change the default behavior
in the long run" patch, opening the discussion whether other people
agree with deprecating the current behavior in favor of the rescheduling
behavior.

But the consensus on the Git mailing list was that it would make sense
to show a warning in the near future, and flip the default
rebase.rescheduleFailedExec to reschedule failed `exec` commands by
default. See e.g.
<CAGZ79kZL5CRqCDRb6B-EedUm8Z_i4JuSF2=UtwwdRXMitrrOBw@mail.gmail.com>

So let's back out that patch that added the `-y` short option that we
agreed was not necessary or desirable.

This reverts commit 81ef8ee75d5f348d3c71ff633d13d302124e1a5e.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>