remove support legacy 7-bit ASCII
libc has support for a 7-bit ASCII mode, which we added from FreeBSD, but which Solaris has never had. We simply do not need this, as 8-bit clean modes include 7-bit ASCII, and as the 7-bit ASCII mode only makes sense in USA where ISO8859-1 or even UTF-8 would be preferable.
Furthermore, there is code in libc which runs at a slight performance penalty because it has to check explicitly for ASCII encoding.
We should just remove this pointless encoding and the associated code in libc.