Revert "Added ItemSetAddEvent and ItemSetRemoveEvent (#2794)"

This reverts commit 85251833de3bd101d388b20fdb9b02c532a9f1c9.

Conflicts:
	server/src/com/vaadin/data/util/AbstractInMemoryContainer.java
	server/tests/src/com/vaadin/data/util/TestIndexedContainer.java

Change-Id: I49dfdc58ae5841892c6d8e205c520437da9a05e5