magit-stash-drop: Don't enable Helm's multi-item selection

magit-stash-drop has let-bound helm-comp-read-use-marked around its
magit-read-stash call since 53f7cfe6 (magit-{tag-delete,stash-drop}:
Supports Helm's marked candidates, 2020-08-29).  However, soon after
that commit, magit-read-stash's processing became incompatible with
reading multiple items rather than one: 7f7c3aa4 (magit-read-stash:
Show the commit messages while reading, 2020-10-20).  The result is
that Helm users see a type error when they call magit-stash-drop from
the "Stashes" section.

Fix the error by removing support for marking multiple candidates.
Instead it'd be possible to adjust magit-read-stash to handle multiple
items, but

  * that's too much special handling for one completion framework.
    The goal with 53f7cfe6 was just to add support for Helm's
    multi-item completion in the spots where it worked with the
    existing code and didn't complicate things on Magit's side.

  * it's unlikely that dropping this functionality affects any users
    because it was broken within a couple months of being added, never
    made it into a release, and has been broken for over a year.

Fixes #4571.

1 files changed, 3 insertions, 0 deletions

diff --git a/docs/RelNotes/3.4.0.org b/docs/RelNotes/3.4.0.org
index ad3d6c4..e305918 100644
--- a/docs/RelNotes/3.4.0.org
+++ b/docs/RelNotes/3.4.0.org
@@ -31,3 +31,6 @@
   visited by a Dired buffer to ~git blame~, leading to some commands
   (e.g., ~magit-checkout~) erroring when called from a Dired buffer.
   #4560
+
+- For Helm users, calling ~magit-stash-drop~ on the "Stashes" section
+  has been broken since v3.0.0.  #4571