Refactor away getEmbedDialog() from EntityEmbedTestBase, just use URLs

To simplify the tests and make them easier to read, I suggest we drop the getEmbedDialog helper function.

 
-  /**
-   * Retrieves an embed dialog based on given parameters.
-   *
-   * @param string $filter_format_id
-   *   ID of the filter format.
-   * @param string $embed_button_id
-   *   ID of the embed button.
-   *
-   * @return string
-   *   The retrieved HTML string.
-   */
-  public function getEmbedDialog($filter_format_id = NULL, $embed_button_id = NULL) {
-    $url = 'entity-embed/dialog';
-    if (!empty($filter_format_id)) {
-      $url .= '/' . $filter_format_id;
-      if (!empty($embed_button_id)) {
-        $url .= '/' . $embed_button_id;
-      }
-    }
-    return $this->drupalGet($url);
-  }
-
 }

It just obfuscates the url on drupalGet() calls, as it just concatenates a string to create a url that's easy to understand by looking at the url itself.

I think it's clearer to see the url in the test:

-    $this->getEmbedDialog('custom_format', 'node');
+    $this->drupalGet('/entity-embed/dialog/custom_format/node');