rebase: remove obsolete and unused LONG_USAGE which breaks xgettext

rebase: remove obsolete and unused LONG_USAGE which breaks xgettext

Since there is a modern OPTIONS_SPEC variable in use in this script,
the obsolete USAGE and LONG_USAGE variables are no longer used.
Remove them.

In addition, the obsolete LONG_USAGE variable has the following
message in it:

    A'\''--B'\''--C'\''

And such complex LONG_USAGE message will break xgettext when
extracting l10n messages (but if single quotes are removed from the
message, xgettext works fine on 'git-rebase.sh').

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Reviewed-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>