Implement db.Erase(Key) for memoria.

[0xnullifier]

Currently only Read and Write functionality are there in memoria. This issue implements a new feature of Erase which is as simple as it sounds just remove the key, value pair.

Yes just simply remove the file form file system using the os package! remember to remove all the directories in the PathKey for the key.

This problem is marked Easy for the naive implementation i.e no error handling. with proper error handling you get points for Medium

NOTE : always lock the mutex and defer unlock the mutex when doing any file i/o in memoria