docs(guide/directive): remove confusing best practice

Prior to this point, the docs do not mention returning a postLink function, and all the examples use
the definition object form. So, this message is confusing to new readers who may misinterpret
"returning a function" as returning the factory function mentioned in the previous section.

Since this page is only a "gentle introduction" to directives, and using the definition object is a
best practice, it is best to just remove the message altogether.

Closes #14871

docs/content/guide/directive.ngdoc

@@ -162,10 +162,6 @@ initialization work here. The function is invoked using
 {@link auto.$injector#invoke $injector.invoke} which makes it injectable just like a
 controller.
 
-<div class="alert alert-success">
-**Best Practice:** Prefer using the definition object over returning a function.
-</div>
-
 
 We'll go over a few common examples of directives, then dive deep into the different options
 and compilation process.