Remove now-useless XRegExp.exec tests
Remove now-useless XRegExp.exec tests

See https://github.com/slevithan/xregexp/pull/207#discussion_r159169226

slevithan/xregexp,mathiasbynens/xregexp,mathiasbynens/xregexp,GerHobbelt/xregexp,mathiasbynens/xregexp,GerHobbelt/xregexp,GerHobbelt/xregexp,mathiasbynens/xregexp,slevithan/xregexp,slevithan/XRegExp,slevithan/xregexp,slevithan/XRegExp