Revert r41728

* string.c (rb_str_inspect): use encoding index macros in encdb.h.

It breaks build because encdb.h requires miniruby and modified
miniruby requires encdb.h.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41729 b2dd03c8-39d4-4d8f-98ff-823fe69b080e