[chore] Fix a few luacheck errors (#8460)

Inadvertently introduced in <https://github.com/koreader/koreader/pull/8432> and <https://github.com/koreader/koreader/pull/8445>.

frontend/apps/reader/modules/readerhighlight.lua

@@ -1308,7 +1308,7 @@ function ReaderHighlight:onHoldRelease()
         end
         return true
     end
-    
+
     local long_final_hold = false
     if self.hold_last_tv then
         local hold_duration = TimeVal:now() - self.hold_last_tv

frontend/apps/reader/modules/readerpaging.lua

@@ -4,7 +4,6 @@ local Event = require("ui/event")
 local Geom = require("ui/geometry")
 local InputContainer = require("ui/widget/container/inputcontainer")
 local Math = require("optmath")
-local Notification = require("ui/widget/notification")
 local ReaderZooming = require("apps/reader/modules/readerzooming")
 local TimeVal = require("ui/timeval")
 local UIManager = require("ui/uimanager")

frontend/apps/reader/modules/readerrolling.lua

@@ -4,7 +4,6 @@ local ConfirmBox = require("ui/widget/confirmbox")
 local Device = require("device")
 local Event = require("ui/event")
 local InputContainer = require("ui/widget/container/inputcontainer")
-local Notification = require("ui/widget/notification")
 local ProgressWidget = require("ui/widget/progresswidget")
 local ReaderPanning = require("apps/reader/modules/readerpanning")
 local Size = require("ui/size")