tests: domainschemadata: Drop redundant tests

tests: domainschemadata: Drop redundant tests

The schema tested by removed test cases is tested by other, more useful,
test cases:

 - 'maxMemory'
    - qemuxmlargvdata/memory-hotplug*
 - 'backingChains'
    - qemuxmlargvdata/disk-backing-chains*
 - 'timers'
    - qemuxml2argvdata/kvm-pit-delay.xml
    - qemuxml2argvdata/clock-catchup.xml
 - 'qemu-simple-description-title.xml'
    - 'qemuxml2argvdata/minimal.xml

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>