InputContainer/FocusManager: Simplify key_events clearing on unplug

Thanks to @poire-z for the idea, it's indeed much nicer this way ;).

frontend/apps/reader/modules/readertoc.lua

@@ -35,7 +35,7 @@ local ReaderToc = InputContainer:extend{
 }
 
 function ReaderToc:init()
-    self:registerKeyEvents(true)
+    self:registerKeyEvents()
 
     if G_reader_settings:hasNot("toc_items_per_page") then
         -- The TOC items per page and items' font size can now be
@@ -60,16 +60,13 @@ end
 
 function ReaderToc:onGesture() end
 
-function ReaderToc:registerKeyEvents(init)
+function ReaderToc:registerKeyEvents()
     if Device:hasKeyboard() then
         self.key_events.ShowToc = { { "T" } }
-    elseif not init then
-        self.key_events.ShowToc = nil
     end
 end
 
 ReaderToc.onPhysicalKeyboardConnected = ReaderToc.registerKeyEvents
-ReaderToc.onPhysicalKeyboardDisconnected = ReaderToc.registerKeyEvents
 
 function ReaderToc:onReadSettings(config)
     self.toc_ticks_ignored_levels = config:readSetting("toc_ticks_ignored_levels") or {}