conf: remove pointless check on enum value

conf: remove pointless check on enum value

'val' is initialized from virDomainCapsFeatureTypeFromString and a
few lines earlier there was already a check for 'val < 0'.

The 'val >= 0' is thus always true. The enum conversion similarly
ensures that the val will be less than VIR_DOMAIN_CAPS_FEATURE_LAST,
so "val < VIR_DOMAIN_CAPS_FEATURE_LAST' is thus always true too.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>