Fix warning about Drop trait used as bound

Removes the following warning:

warning: bounds on Self: Drop are useless, consider instead using std::mem::needs_drop to detect if a type has a destructor
  --> gdnative-core\src\core_types\access.rs:36:25
   |
36 | pub unsafe trait Guard: Drop + private::Sealed {
   |                         ^^^^
   |
   = note: #[warn(drop_bounds)] on by default

Author: Bromeon

gdnative-core/src/core_types/access.rs

@@ -33,7 +33,7 @@ where
 ///
 /// This trait is sealed and has no public members.
 #[allow(clippy::len_without_is_empty)]
-pub unsafe trait Guard: Drop + private::Sealed {
+pub unsafe trait Guard: private::Sealed {
     #[doc(hidden)]
     type Target;