Remove E-rater grammar service

[jrchamp]

From https://guides.turnitin.com/hc/en-us/articles/27251688507533-Turnitin-release-notes#h_01JFJZMSQ0Z16ZVCYMQV4HNQ4Z

Educational Testing Services (ETS) will no longer be supporting their e-rater grammar service in 2025. As a result, Turnitin has removed e-rater from all of its products.

I don't fully understand the scope on whether this affects all of E-rater or just the grammar portion, but the now-broken grammar portion is definitely causing confusion for some of our users.

moodle-mod_turnitintooltwo/mod_form.php

Lines 607 to 609 in e70782a

	$mform->addElement('checkbox', 'erater_grammar', '', " ".get_string('erater_grammar', 'turnitintooltwo'));
	$mform->setDefault('erater_grammar', $config->default_grammar_grammar);
	$mform->disabledIf('erater_grammar', 'erater', 'eq', 0);