BBB hard-coded dependency on mod_lti

See: https://github.com/blindsidenetworks/moodle-mod_bigbluebuttonbn/blob/50b31b851b645eb167b0c4958dc601decc02fc50/lib.php#L33

I couldn't find any reference to LTI specific code other than in comments in the two task classes that appear to be copy/paste errors

Regardless this dependency shouldn't be hardcoded in lib.php as it will cause any site with BBB installed, without the LTI module, to break