engine, restapi: Remove redundant SizeConverter

Engine has 2 implementations of SizeConverter utility which converts
disk size from one unit to another. One of those implementations
resides in the common code and its code looks elegant therefore its
chosen to stay while other has been deleted.

Change-Id: I6a82a4a2ac199eb226b8848ac579106ca0497669
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>