Remove dead unitframes showtooltip setting

The mouseover-scripts migration dropped the unit frame OnEnter/OnLeave
handlers that read C.unitframes[unit].showtooltip, so nothing consulted the
setting anymore. The engine mouseover (driven by the frame's unit attribute)
now shows the native tooltip unconditionally, so the option could not gate it
even if a reader remained.

Drop the config default, the "Enable Mouseover Tooltip" GUI checkbox, and the
now-orphaned locale string from all translation files.

env/translations_enUS.lua

@@ -293,7 +293,6 @@ pfUI_translation["enUS"] = {
   ["Enable Mana Ticks"] = nil,
   ["Enable Micro Bar"] = nil,
   ["Enable Mouselook With Right Click"] = nil,
-  ["Enable Mouseover Tooltip"] = nil,
   ["Enable Movable Bags"] = nil,
   ["Enable Offscreen Frame Positions"] = nil,
   ["Enable Overlap"] = nil,