util: remove unused functions from virnetdev.c
    
    The global functions virNetDevReplaceMacAddress(),
    virNetDevReplaceNetConfig(), virNetDevRestoreMacAddress(), and
    virNetDevRestoreNetConfig() are no longer used, as their functionality
    has been replaced by virNetDev(Save|Read|Set)NetConfig().
    
    The static functions virNetDevReplaceVfConfig() and
    virNetDevRestoreVfConfig() were only used by the above-named global
    functions that were removed.