merge giveUpFocus into unfocusIfFocused

in fact, the old `unfocusIfFocused` has been broken for a while,
since hud iframe is inside a shadowRoot on Firefox 63+.

Author: gdh1995

content_scripts/hud.coffee

@@ -111,9 +111,6 @@ HUD =
     @pasteListener data
 
   unfocusIfFocused: ->
-    document.activeElement.blur() if document.activeElement == @hudUI?.iframeElement
-
-  giveUpFocus: ->
     # On Firefox, if an <iframe> disappears when it's focused, then it will keep "focused",
     # which means keyboard events will always be dispatched to the HUD iframe
     if @hudUI?.showing

content_scripts/mode_find.coffee

@@ -79,7 +79,7 @@ class FindMode extends Mode
     HUD.showFindMode this
 
   exit: (event) ->
-    HUD.giveUpFocus()
+    HUD.unfocusIfFocused()
     super()
     FindMode.handleEscape() if event