Deprecation Guide for Remove All Listeners/Observers

When using both the removeListener and removeObserver methods, users can omit the final string or method argument to trigger an undocumented codepath that will remove all event listeners/observers for the given key:

let foo = {
  method1() {}
  method2() {}
};

addListener(foo, 'init', 'method1');
addListener(foo, 'init', 'method2');

removeListener(foo, 'init');

This functionality will be removed since it is uncommonly used, undocumented, and adds a fair amount of complexity to a critical path. To update, users should remove each listener individually:

let foo = {
  method1() {}
  method2() {}
};

addListener(foo, 'init', 'method1');
addListener(foo, 'init', 'method2');

removeListener(foo, 'init', 'method1');
removeListener(foo, 'init', 'method2');