[Biki]

ZO_PlayerAttributeHealthBarLeftGloss:SetHandler("OnUpdate", function() 
    ZO_PlayerAttributeHealthBarLeftGloss:SetHidden(true)
  end)
  ZO_PlayerAttributeHealthBarRightGloss:SetHandler("OnUpdate", function() 
    ZO_PlayerAttributeHealthBarRightGloss:SetHidden(true)
  end)

Will work too, quick and dirty. Just tested it. But there might be more performant options available.