rb_check_safe_obj no longer exists

Commit e91c39f1c0 deleted definition of it.
Though I'm not sure if we can delete public API like this, it no longer
works nontheless.  Having declaration without definiton is worse than
having nothing at all.  Just delete the declartion too.

include/ruby/ruby.h

@@ -603,7 +603,6 @@ char *rb_string_value_cstr(volatile VALUE*);
 #define StringValuePtr(v) rb_string_value_ptr(&(v))
 #define StringValueCStr(v) rb_string_value_cstr(&(v))
 
-void rb_check_safe_obj(VALUE);
 #define SafeStringValue(v) StringValue(v)
 #if GCC_VERSION_SINCE(4,4,0)
 void rb_check_safe_str(VALUE) __attribute__((error("rb_check_safe_str() and Check_SafeStr() are obsolete; use StringValue() instead")));