[3.12] gh-113951: Tkinter: "tag_unbind(tag, sequence, funcid)" now only unbinds "funcid" (GH-113955) (GH-114997)

Previously, "tag_unbind(tag, sequence, funcid)" methods of Text and
Canvas widgets destroyed the current binding for "sequence", leaving
"sequence" unbound, and deleted the "funcid" command.

Now they remove only "funcid" from the binding for "sequence", keeping
other commands, and delete the "funcid" command.
They leave "sequence" unbound only if "funcid" was the last bound command.
(cherry picked from commit 7e42fddf608337e83b30401910d76fd75d5cf20a)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>