remove unnecessary declaration so that rdoc works

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53852 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -1363,7 +1363,6 @@ rb_str_resurrect(VALUE str)
  *  times (and call many realloc).
  */
 
-static inline void str_modifiable(VALUE str);
 static VALUE
 rb_str_init(int argc, VALUE *argv, VALUE str)
 {