Remove deprecated functions in WebTestBase.php

Follow up from: #2421699: Remove usage of deprecated functions of WebTestBase.

Remove the following deprecated functions:

  /**
   * Sets the raw HTML content.
   *
   * @deprecated 8.x
   *   Use setRawContent().
   */
  protected function drupalSetContent($content) {
    $this->setRawContent($content);
  }

  /**
   * Sets the value of drupalSettings for the currently-loaded page.
   *
   * @deprecated 8.x
   *   Use setDrupalSettings().
   */
  protected function drupalSetSettings($settings) {
    $this->setDrupalSettings($settings);
  }

  /**
   * Gets the current raw HTML of requested page.
   *
   * @deprecated 8.x
   *   Use getRawContent().
   */
  protected function drupalGetContent() {
    return $this->getRawContent();
  }

  /**
   * Gets the value of drupalSettings for the currently-loaded page.
   *
   * @deprecated 8.x
   *   Use getDrupalSettings().
   */
  protected function drupalGetSettings() {
    return $this->getDrupalSettings();
  }