Remove Lock structure again as we changed design

src/memory/paging/mod.rs

@@ -1,8 +0,0 @@
-/// The paging lock must be unique. It is required for all page table operations and thus
-/// guarantees exclusive page table access.
-pub struct Lock {
-    _private: (),
-}
-
-impl !Send for Lock {}
-impl !Sync for Lock {}