Fixup MinMaxElementsValidation with disappearances
When a list disappears, even when it has a certain number of minimum
elements, MinMaxElementsValidation should not take any action.
The resposibility for enforcing the list being present when it has
min-elements > 0 falls under perview of MandatoryNodeValidator, which
covers the non-existence of the list at the proper enforcement point
-- something MinMaxElementsValidation cannot do.
JIRA: YANGTOOLS-909
Change-Id: I3612a1c1461456b2bd41651decab1032581f85d1
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>