Fix race condition in retrieveFileList()

[Botono]

I have been getting a fatal error: sync: unlock of unlocked mutex when using the package in my setup, although I have not been able to reproduce it in isolation yet. However, implementing this change fixes the panic.

Unlocking the mutex inside of a locking method introduces a race condition. This change creates private methods that do the operations of the Remove() and RemoveRecursive() methods, but without the mutex locks. This allows retrieveFileList() to call them without unlocking the mutex first.

Remove() and RemoveRecursive() are then updated to call the private methods instead of doing the processing themselves.