doc/git-repack: don't mention nonexistent "--unpacked" option

The documentation for geometric repacking mentions a "--unpacked" option
that supposedly changes how loose objects are rolled up. This option has
never existed, and the implied behaviour, namely to include all unpacked
objects into the resulting packfile, is in fact the default behaviour.

Correct the documentation to not mention this option.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

Documentation/git-repack.txt

@@ -203,11 +203,8 @@ uniquely by the set of packs being "rolled-up"; in other words, the
 packs determined to need to be combined in order to restore a geometric
 progression.
 +
-When `--unpacked` is specified, loose objects are implicitly included in
-this "roll-up", without respect to their reachability. This is subject
-to change in the future. This option (implying a drastically different
-repack mode) is not guaranteed to work with all other combinations of
-option to `git repack`.
+Loose objects are implicitly included in this "roll-up", without respect to
+their reachability. This is subject to change in the future.
 +
 When writing a multi-pack bitmap, `git repack` selects the largest resulting
 pack as the preferred pack for object selection by the MIDX (see