Route hooksecurefunc callers through pfUI.hooksecurefunc; global belongs to ClassicAPI

pfUI's Lua hooksecurefunc lived in pfUI.env and shadowed ClassicAPI's C
global for all pfUI code. Replace it with a thin pfUI.hooksecurefunc shim
that keeps the missing-target no-op our call sites rely on (ClassicAPI
errors on a nil target) and delegates the actual hook to _G.hooksecurefunc.

Migrated all 70 internal call sites (modules/libs/skins) to
pfUI.hooksecurefunc; bare hooksecurefunc now resolves to ClassicAPI's C
version everywhere. Dropped the unused prepend path and the orphaned
pfUI.hooks table.

modules/eqcompare.lua

@@ -139,7 +139,7 @@ pfUI:RegisterModule("eqcompare", function ()
 
   local function HookTooltip(tooltip)
     for setter, getter in pairs(TooltipHooks) do
-      _G['hooksecurefunc'](tooltip, setter, makeHook(getter))
+      pfUI.hooksecurefunc(tooltip, setter, makeHook(getter))
     end
   end