MDL-50287 lang: remove unused string

(I don't think this string warrants deprecation)

lang/en/admin.php

@@ -1019,7 +1019,6 @@
 $string['taskcachecron'] = 'Background processing for caches';
 $string['taskcalendarcron'] = 'Send calendar notifications';
 $string['taskcheckforupdates'] = 'Check for updates';
-$string['taskcompletioncron'] = 'Calculate completion data';
 $string['taskcompletionregular'] = 'Calculate regular completion data';
 $string['taskcompletiondaily'] = 'Completion mark as started';
 $string['taskcontextcleanup'] = 'Cleanup contexts';