Remove USE_SELVERSION variable after upgrading to Selenium 4

[NoelDeMartin]

As mentioned in #113, once Selenium 4 is used the USE_SELVERSION variable won't be necessary anymore. This variable was introduced because the variable SELVERSION is currently ignored when using Chrome.

[stronk7]

To avoid forgetting, we also have to remove it from https://git.in.moodle.com/integration/nightlyjobs