🔥 Remove broken Dock::observeTextEditors(callback) fn

We're updating Atom to disallow editors in docks. As a result, we need
to remove Dock::observeTextEditors(callback). Normally, we would
deprecate a method before removing it, but this method is broken and has
never worked:

  atom.workspace.getLeftDock().observeTextEditors(console.log)
  (unknown) Uncaught TypeError: this.paneContainer.getTextEditors is not a function
      at Dock.getTextEditors (/Applications/Atom.app/Contents/Resources/app/src/dock.js:590:37)
      at Dock.observeTextEditors (/Applications/Atom.app/Contents/Resources/app/src/dock.js:396:41)
      at <anonymous>:1:30
  getTextEditors @ <embedded>:29933
  observeTextEditors @ <embedded>:29739
  (anonymous) @ VM1941:1

Since the method is broken, we know that nobody is relying on it.
Instead of deprecating the method, we can just remove it.

src/dock.js

@@ -385,21 +385,6 @@ module.exports = class Dock {
   Section: Event Subscription
   */
 
-  // Essential: Invoke the given callback with all current and future text
-  // editors in the dock.
-  //
-  // * `callback` {Function} to be called with current and future text editors.
-  //   * `editor` An {TextEditor} that is present in {::getTextEditors} at the time
-  //     of subscription or that is added at some later time.
-  //
-  // Returns a {Disposable} on which `.dispose()` can be called to unsubscribe.
-  observeTextEditors (callback) {
-    for (const textEditor of this.getTextEditors()) {
-      callback(textEditor)
-    }
-    return this.onDidAddTextEditor(({textEditor}) => callback(textEditor))
-  }
-
   // Essential: Invoke the given callback with all current and future panes items
   // in the dock.
   //