Remove "Properly Unicode safe tokeniser and pads" from the todo

Remove "Properly Unicode safe tokeniser and pads" from the todo

Brian Fraser (during GSoC), Father Chrysostomos, Zefram and others have done
most of the tasks suggested in this todo, so remove it from consideration as
it's clearly stale. The only possible work outstanding is how toke.c behaves
when PL_rsfp is marked as returning UTF-8, and when source filters return
SVs with SvUTF8() true. So leave this in as a comment in the Pod so that it
does not get lost, pending proper investigation.