conf: remove unused VIR_ENUM_DECL

While adding a new enum, I noticed a VIR_ENUM_DECL for a type that
doesn't exist. There is also of course no matching VIR_ENUM_IMPL for
it.

src/conf/domain_conf.h

@@ -1830,7 +1830,6 @@ VIR_ENUM_DECL(virDomainLifecycle)
 VIR_ENUM_DECL(virDomainLifecycleCrash)
 VIR_ENUM_DECL(virDomainDevice)
 VIR_ENUM_DECL(virDomainDeviceAddress)
-VIR_ENUM_DECL(virDomainDeviceAddressMode)
 VIR_ENUM_DECL(virDomainDisk)
 VIR_ENUM_DECL(virDomainDiskDevice)
 VIR_ENUM_DECL(virDomainDiskBus)