Clippy nightly: fix lifetime annotations not used in the impl block

Original errors (4):
-------------------------------------------------------------------------------
 error: this lifetime isn't used in the impl
   --> gdnative-core/src/core_types/variant/serialize.rs:114:6
    |
114 | impl<'d> Serialize for DictionaryDispatch {
    |      ^^
    |
    = note: `-D clippy::extra-unused-lifetimes` implied by `-D clippy::complexity`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_lifetimes
-------------------------------------------------------------------------------

Author: Bromeon

gdnative-core/src/core_types/variant/serialize.rs

@@ -111,7 +111,7 @@ struct DictionaryDispatchEntry {
     value: VariantDispatch,
 }
 
-impl<'d> Serialize for DictionaryDispatch {
+impl Serialize for DictionaryDispatch {
     fn serialize<S>(&self, serializer: S) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>
     where
         S: Serializer,

gdnative-core/src/object/as_arg.rs

@@ -44,7 +44,7 @@ mod private {
 }
 
 // Null
-impl<'a, T> private::Sealed for Null<T> {}
+impl<T> private::Sealed for Null<T> {}
 
 // Temporary references (shared ownership)
 impl<'a, T: GodotObject> private::Sealed for TRef<'a, T, Shared> {}
@@ -59,14 +59,14 @@ impl<T: NativeClass, Own: Ownership> private::Sealed for Instance<T, Own> {}
 // ----------------------------------------------------------------------------------------------------------------------------------------------
 // Null
 
-impl<'a, T: GodotObject> AsArg<T> for Null<T> {
+impl<T: GodotObject> AsArg<T> for Null<T> {
     #[inline]
     fn as_arg_ptr(&self) -> *mut sys::godot_object {
         std::ptr::null_mut()
     }
 }
 
-impl<'a, T: GodotObject> AsVariant for Null<T> {
+impl<T: GodotObject> AsVariant for Null<T> {
     type Target = T;
 }