engine: Remove redundant constant IrsBrokerCommand.BYTES_TO_GB

We have a lot of similar constants across the Engine. BYTES_TO_GB is
one of them. Generic org.ovirt.engine.core.common.utils.SizeConverter
introduces BYTES_IN_GB which can be used instead of
IrsBrokerCommand.BYTES_TO_GB.

Change-Id: I0a3cede172aebab1182dca0ceec901be8553e6ae
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>