Remove unused Init_frozen_strings declaration

Init_frozen_strings definition is removed in r51511.
https://bugs.ruby-lang.org/issues/11423

Patch by Kohei Suzuki <eagletmt@gmail.com>

* internal.h: Remove declaration of unexist function
  [Fix GH-1558]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58205 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

internal.h

@@ -1536,7 +1536,6 @@ VALUE rb_strftime(const char *format, size_t format_len, rb_encoding *enc,
 #endif
 
 /* string.c */
-void Init_frozen_strings(void);
 VALUE rb_fstring(VALUE);
 VALUE rb_fstring_new(const char *ptr, long len);
 #define rb_fstring_lit(str) rb_fstring_new((str), rb_strlen_lit(str))