SouthboundIT: drop test case 4

As implemented, test case 4 is equivalent to test case 3. Modifying
test case 4 so it matches the documentation (i.e., setting the value
but not the key) just causes errors in MD-SAL because key-less values
are not allowed. This patch drops test case 4 entirely.

Change-Id: Id52eeddd1c7a1660a1da3ce82e31ac7295bff635
Signed-off-by: Stephen Kitt <skitt@redhat.com>