Remove useless methods in DbCommand class

Why is this change necessary?
Since DbCommand objects are used for storing unsynced (with Google
Calendar) commands, there is no need for methods like setTask(); the
task associated with the command (add/delete) should be initialised from
the constructor and remain unchanged. Also, I'm thinking of using
polymorphism for getting different behaviours out of add and delete
commands, so there's no need for the commandType field.

What side effects does this change have?
Implement polymorphism for different types of commands, specifically add
and delete commands for now, for the unsynced command stack of database.