* enc/unicode.c: Remove special processing for U+03B9/U+03BC/U+A64B
  (GREEK SMALL LETTERs IOTA/MU, CYRILLIC SMALL LETTER MONOGRAPH UK)
  from onigenc_unicode_case_map and simplify code.

* enc/unicode/case-folding.rb: Remove check for U+03B9/U+03BC/U+A64B.

This and the previous few related commits make sure that we won't hit
the equivalent of bug #12990 anymore for future updates of Unicode versions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56976 b2dd03c8-39d4-4d8f-98ff-823fe69b080e