encdb.c, encoding.c: make BOM-encodings dummy

* enc/encdb.c (ENC_DUMMY_UNICODE): make BOM-encodings dummy.
* encoding.c (enc_autoload): keep dummy encodings dummy.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43034 b2dd03c8-39d4-4d8f-98ff-823fe69b080e