Cast bars: fill and count down from duration objects, drop the OnUpdate sweeps

Player/target/focus bars and nameplate castbars hand their fill to
StatusBar:SetTimerDuration and their timer text to a duration text binding.
Pushback re-reads UnitCastingDuration/UnitChannelDuration; a C_Timer clears a
bar whose STOP never arrives. The nameplate castbar throttle is obsolete and
goes with its settings page.

env/translations_enUS.lua

@@ -939,7 +939,6 @@ pfUI_translation["enUS"] = {
   ["Chat Tab"] = nil,
   ["Libpredict"] = nil,
   ["Panel Alignment"] = nil,
-  ["Nameplate Update Rate"] = nil,
   ["Target/Casting Plates"] = nil,
   ["Tooltip Update Rate"] = nil,
   ["Cursor Follow"] = nil,