mouseover/libpredict: drop legacy spell-target plumbing

With Nampower as a hard dep, /pfcast for spell names always takes the
early CastSpellByName(msg, unit) path. The fallback branch that did the
SpellTargetUnit dance (resolve a friendly unit token, disable AutoSelf
Cast, call SpellTargetUnit) hasn't been reachable in a while, and it
dragged a pile of supporting infrastructure with it.

modules/mouseover.lua:
- Drop the st_units token list, GetUnitString helper, and the
  UnitTokenFromGUID rewrite of GetUnitString — all only used by the
  dead fallback.
- Drop the NoSelfCast helper (only the dead fallback called it).
- Drop the pfMouseOver frame; its only purpose was to hold a .unit
  field the dead fallback wrote and libpredict's hook read.
- The macro path collapses to: if not the current target, swap target,
  run the loadstring'd func, restore the previous target.
- 99 lines → 34.

libs/libpredict.lua:
- Drop the dead `local mouseover = pfUI.uf.mouseover.unit` plumbing in
  the CastSpellByName hook — pfUI.uf.mouseover is gone and the field
  was permanently nil anyway. The three `target or mouseover or default`
  fallback chains collapse to `target or default`.

Modern mouseover/click-to-cast detection in libpredict goes through
pfUI.libpredict_pending_cast (populated by libdebuff from Nampower's
SPELL_CAST_EVENT) — that path is GUID-based, server-authoritative, and
untouched.

libs/libpredict.lua

@@ -959,8 +959,6 @@ hooksecurefunc("CastSpellByName", function(effect, target)
   if not libpredict.sender.enabled then return end
   local effect, rank = libspell.GetSpellInfo(effect)
   if not effect then return end
-  local mouseover = pfUI and pfUI.uf and pfUI.uf.mouseover and pfUI.uf.mouseover.unit
-  mouseover = mouseover and UnitCanAssist("player", mouseover) and UnitName(mouseover)
 
   local default = UnitName("target") and UnitCanAssist("player", "target") and UnitName("target") or UnitName("player")
 
@@ -979,13 +977,13 @@ hooksecurefunc("CastSpellByName", function(effect, target)
   if not libpredict.sender.current_cast then
     spell_queue[1] = effect
     spell_queue[2] = effect.. ( rank or "" )
-    spell_queue[3] = target or mouseover or default
+    spell_queue[3] = target or default
   end
   
   -- Instant HoTs: libdebuff/Nampower via GetHotDuration, hook method as fallback
   
   if effect == REJUVENATION then
-    local hotTarget = target or mouseover or default
+    local hotTarget = target or default
     local now = pfUI.uf.now or GetTime()
     local key = "Reju" .. hotTarget
     
@@ -1002,7 +1000,7 @@ hooksecurefunc("CastSpellByName", function(effect, target)
     local rankStr = rankNum and tostring(rankNum) or "0"
     libpredict.sender:SendHealCommMsg("Reju/"..hotTarget.."/"..rejuvDuration.."/"..rankStr.."/")
   elseif effect == RENEW then
-    local hotTarget = target or mouseover or default
+    local hotTarget = target or default
     local now = pfUI.uf.now or GetTime()
     local key = "Renew" .. hotTarget