docs/git-rebase: remove redundant entry in incompatible options list

The --autosquash option is implied by the earlier --[no-]autosquash
entry in the list.

Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -535,7 +535,6 @@ Flags only understood by the interactive backend:
  * --interactive
  * --exec
  * --keep-empty
- * --autosquash
  * --edit-todo
  * --root when used in combination with --onto