Garbage collect dead code intended to write five- and six-byte UTF-8

Garbage collect dead code intended to write five- and six-byte UTF-8
sequences since the Unicode standard has been explicitly prohibiting
the use of such sequences when encoding Unicode characters for more
than 20 years now.

While here, also weed out UTF-16 surrogates and codepoints in the
invalid range 110000 to 1FFFFF if any are encountered.  I hoped to
write "no functional change", but to my shame it turns out there
are unrelated bugs with \[uXXXX] parsing in roff_escape.c, so this
new anti-surrogate check is actually reachable until those other
bugs get fixed, and even after fixing those other bugs, it will
remain useful as a defense in depth.