core: rm unused method in AddDiskToVmCommandTest

Patch 73499ce148500edc73642fb6c548ad597a932f6e changed the mocking
scheme of this test.
As a result, the method mockStorageDomain(Guid, int, int) remained
unused.

This patch remove the unneeded method.

Change-Id: I5163e6d9615ad3e0c44edcf00d43f3cb04aa3d7c
Signed-off-by: Allon Mureinik <amureini@redhat.com>