Fix warnings in database::abstraction

src/database/abstraction.rs

@@ -23,12 +23,12 @@ pub trait DatabaseEngine: Send + Sync {
     where
         Self: Sized;
     fn open_tree(&self, name: &'static str) -> Result<Arc<dyn Tree>>;
-    fn flush(self: &Self) -> Result<()>;
-    fn cleanup(self: &Self) -> Result<()> {
+    fn flush(&self) -> Result<()>;
+    fn cleanup(&self) -> Result<()> {
         Ok(())
     }
-    fn memory_usage(self: &Self) -> Result<String> {
-        Ok("Current database engine does not support memory usage reporting.".to_string())
+    fn memory_usage(&self) -> Result<String> {
+        Ok("Current database engine does not support memory usage reporting.".to_owned())
     }
 }