core: Remove api's StrigUtils

This patch removes the class
org.ovirt.engine.api.common.util.StringUtils.

It only had one method (isNullOrEmpty(String)), which was only used in
two places.
This method's body is completely identical to
org.apache.commons.lang.StringUtils.isEmpty(String), so it could easily
be replaced.

Change-Id: Ia6f116f861975db24b710db457b40491908ff1d1
Signed-off-by: Allon Mureinik <amureini@redhat.com>