Remove some redundant code from Canvas.tag_bind(), which added all

Remove some redundant code from Canvas.tag_bind(), which added all
bindings to a dictionary _tagcommands which was otherwise unused.
(This was checked in accidentally with rev. 1.125 and not deleted with
rev. 1.127 when the other half of this code was removed -- although
even as originally checked in the _tagcommands variable was never
used.)

(PR#40, reported by Peter Stoehr)