delete useless admin commands

To clear caches, restart the server. We may want to consider adding the
cache sizes and database memory usage as metrics in the future.

src/database/abstraction.rs

@@ -20,10 +20,6 @@ pub(crate) trait KeyValueDatabaseEngine: Send + Sync {
     fn cleanup(&self) -> Result<()> {
         Ok(())
     }
-    fn memory_usage(&self) -> Result<String> {
-        Ok("Current database engine does not support memory usage reporting."
-            .to_owned())
-    }
 }
 
 pub(crate) trait KvTree: Send + Sync {