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

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.