Database fixes and a little cleanup

After deleting the database (on logout), no new one was created because the MALSqlHelper was not recreated for two reasons:
- the MALSqlHelper is designed as singleton (object is retrieved by calling ```getHelper()```), but ```instance``` was not unset after deleting the database
- ```malSqlHelper``` was defined as static in DatabaseManager, so even fixing the first point would not fix the bug, because all instances of DatabaseManager share the same ```malSqlHelper``` which is not recreated if set once.

Now MALSqlHelper has the new function ```deleteDatabase()``` that deletes the database and sets ```instance``` to null so the next call to ```getHelper()``` will recreate the database. Also ```malSqlHelper``` is not defined as static in DatabaseManager.
I'm a little confused actually, because this should have never worked before... but it has somehow.

Also noticed a possible access violation while testing: running NetworkTasks while deleting the database can cause access violations, so cancel running NetworkTasks on logout to make sure that the tasks are not accessing the database when it is deleted.

I've also found some unused MALManager variables and removed them.