Remove any existing IPC hanlders/listeners before assigning new ones inside the downloadsManager (#2493)

* Remove any existing IPC hanlders/listeners before assigning new ones inside the downloadsManager

* Mock more ipx functions during tests

src/main/app/initialize.test.js

@@ -50,6 +50,8 @@ jest.mock('electron', () => ({
         on: jest.fn(),
         handle: jest.fn(),
         emit: jest.fn(),
+        removeHandler: jest.fn(),
+        removeListener: jest.fn(),
     },
     screen: {
         on: jest.fn(),