Remove todo for UTF8 source filters

Remove todo for UTF8 source filters

Source filters don’t really make sense on character streams.  They are
designed for streams of bytes coming straight from a file.  Things
stop making sense if you have ‘use utf8’ in a UTF-8 scalar (does that
mean double-decode?).

It’s for this reason that evalbytes respects source filters, while
eval does not.  (It doesn’t outside the unicode_eval feature, because
it was never really thought about and the implementation didn’t take
it into account, resulting in strange behaviour.  It doesn’t with the
unicode_eval feature, because it was intentionally prohibited.)