Remove code points < 0x80 from character conversion tables.

Remove code points < 0x80 from character conversion tables.

PostgreSQL treats characters with < 0x80 leading byte  as plain ASCII, and
they are not even passed to the conversion routines. There is no point in
having them in the conversion tables.

Everything in the tables were direct ASCII-ASCII mappings, except for two:
* SHIFT_JIS_2004 code point 0x5C (backslash in ASCII) was mapped to Unicode
  YEN SIGN character.
* Unicode 0x5C (backslash again) was mapped to "REVERSE SOLIDUS" in
  SHIFT_JIS_2004

These mappings never had any effect, so there's no functional change from
removing them.

Discussion: https://postgr.es/m/08e7892a-d55c-eefe-76e6-7910bc8dd1f3@iki.fi