backend: Remove duplicate read of TimeZone in OvfVmReader

Removed duplicate read of TimeZone ovf-field which was not sanitized
against empty strings causing creation of Vm with invalid timezone
"" (correctly should be normalized to null).

Also removed the temporary fix in TimeZoneValidator since now all cases
should be covered and the empty string time-zone always normalized to
null.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1015049
Change-Id: I9152bc347a5358ca794ee8c237e4486f444862ee
Signed-off-by: Martin Betak <mbetak@redhat.com>