rb_sym_interned_p: doesn't exist

Deleted decades ago in commit 6e0fed2

Note also that we eventually ended up reinventing this exact same
functionality.  It is called rb_check_id() now.

include/ruby/internal/intern/parse.h

@@ -50,7 +50,6 @@ int rb_is_local_id(ID);
 RBIMPL_ATTR_CONST()
 int rb_is_junk_id(ID);
 int rb_symname_p(const char*);
-int rb_sym_interned_p(VALUE);
 VALUE rb_backref_get(void);
 void rb_backref_set(VALUE);
 VALUE rb_lastline_get(void);