core: Remove not used ExtendVolume.

Removed ExtendVolumeVDSCommand.
There is no use of it in the engine code.
VDSM side should be re-examined.

Change-Id: I7d9eb342e4ab09c87bc30544cc0820b50cc0c692
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>