Implement delete task from temporary memory

Issue #74 - https://github.com/cs2103aug2014-f11-3j/main/issues/74

Why is this change necessary?
Deleting a task consists of the following parts:
- Delete task from temporary memory
- Delete task from task log

How does it address the issue?
Implementing a method to delete task from temporary memory addresses the
first point of the previous paragraph.

What side effects does this change have?
Logic can call this delete method to delete a task based on its unique
title from temporary memory.