Revert r37098

This reverts
* encoding.c (rb_enc_unicode_p): oniguruma provides Unicode flag.
because UTF-16 and UTF-32 is also treated as unicode in this context.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37104 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

encoding.c

@@ -458,7 +458,8 @@ enc_ascii_compatible_p(VALUE enc)
 int
 rb_enc_unicode_p(rb_encoding *enc)
 {
-    return ONIGENC_IS_UNICODE(enc);
+    const char *name = rb_enc_name(enc);
+    return name[0] == 'U' && name[1] == 'T' && name[2] == 'F' && name[4] != '7';
 }
 
 static st_data_t