Remove useless mbstring encoding 'CP50220-raw'

Remove useless mbstring encoding 'CP50220-raw'

CP50220 is a variant of ISO-2022-JP invented by MicroSoft, which handles some
Unicode characters which are not representable in ISO-2022-JP by converting
them to similar characters which are representable.

What, then, is CP50220-raw? An Internet search turns up absolutely nothing.
Reference works which I consulted don't say anything about it. Other text
conversion libraries don't support it.

From looking at the code: It's just the same as CP50220, but it accepts
unmapped JIS X 0208 characters passed through from other Japanese encodings
and silently encodes them using the usual ISO-2022-JP escape sequence and
representation for JIS X 0208 characters.

It's hard to see how this could be useful. OK, let me come out and say it:
it's _not_ useful. We can confidently jettison this (mis)feature.