Remove references to "Google CSE Advanced"

Problem/Motivation

There is a configuration option at the bottom of the form for the Google CSE Search plugin that offers a checkbox labeled "Use advanced, ad-free version, search engine (You will need a paid account with Google. If enabled, search results will be fetch using Adv engine."

There are two problems with this:

1. The "Adv engine" most likely refers to the XML results API for Google Site Search, which has since been retired (see https://stackoverflow.com/a/45902032. The closest current equivalent is the Custom Search JSON API, which is not currently supported by this module (see #3156181: Add support for Site Restricted Search API).
2. The use_adv configuration parameter that is set by this form input does not have any functional effect in the rest of the module. See comment on line 172 of src/Plugin/Search/GoogleCSESearch.php:

// @todo $condition is an unused variable verify and remove it.

Proposed resolution

Remove the option on the plugin configuration form, and remove all references in code to the use_adv configuration value.

User interface changes

• Remove "Google CSE Advanced" section from plugin configuration form

Data model changes

• Remove use_adv configuration key