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/branches/ruby_2_3@59548 b2dd03c8-39d4-4d8f-98ff-823fe69b080e