Simplify isReaderActivity bookkeeping

src/activities/reader/EpubReaderActivity.h

@@ -203,7 +203,6 @@ class EpubReaderActivity final : public Activity {
   void onExit() override;
   void loop() override;
   void render(RenderLock&& lock) override;
-  bool isReaderActivity() const override { return true; }
   void onButtonAction(CrossPointSettings::BUTTON_ACTION action) override;
 
   // Renders the last saved page to the frame buffer without flushing to display.

src/activities/reader/XtcReaderChapterSelectionActivity.h

@@ -23,5 +23,4 @@ class XtcReaderChapterSelectionActivity final : public Activity {
   void onExit() override;
   void loop() override;
   void render(RenderLock&&) override;
-  bool isReaderActivity() const override { return true; }
 };