git-rebase.txt: update to reflect merge now implemented on sequencer

Since commit 8fe9c3f2 (Merge branch 'en/rebase-merge-on-sequencer',
2019-02-06), --merge now uses the interactive backend (and matches its
behavior) so there is no separate merge backend anymore.  Fix an
oversight in the docs that should have been updated with the previous
change.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -549,8 +549,6 @@ commit started empty (had no changes relative to its parent to
 start with) or ended empty (all changes were already applied
 upstream in other commits).
-The merge backend does the same.
-
 The interactive backend drops commits by default that
 started empty and halts if it hits a commit that ended up empty.
 The `--keep-empty` option exists for the interactive backend to allow