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/readerpanning.lua

@@ -13,14 +13,14 @@ local ReaderPanning = InputContainer:extend{
 }
 
 function ReaderPanning:init()
-    self:registerKeyEvents(true)
+    self:registerKeyEvents()
     -- NOP our own gesture handling
     self.ges_events = nil
 end
 
 function ReaderPanning:onGesture() end
 
-function ReaderPanning:registerKeyEvents(init)
+function ReaderPanning:registerKeyEvents()
     if Device:hasKeyboard() then
         self.key_events = {
             -- these will all generate the same event, just with different arguments
@@ -45,13 +45,10 @@ function ReaderPanning:registerKeyEvents(init)
                 args = {1,  0}
             },
         }
-    elseif not init then
-        self.key_events = {}
     end
 end
 
 ReaderPanning.onPhysicalKeyboardConnected = ReaderPanning.registerKeyEvents
-ReaderPanning.onPhysicalKeyboardDisconnected = ReaderPanning.registerKeyEvents
 
 function ReaderPanning:onPanning(args, _)
     local dx, dy = unpack(args)