remove unneccessary duplication
Add dummy (undocumented) TclCanceled function in stub table (not exported as symbol or macro), which always returns TCL_OK. Needed for Tk 8.5.14 when running in Tcl 8.6 for properly clean-up when a (Tcl 8.6) thread is canceled.
Undo [6a9ee3273c]. Last commit in Tk's core-8-5-branch makes this change no longer necessary.
Merge 8.5.  Tag RC1.