Note, in more recent versions of Moodle, you may be able to do this using the "Custom configuration" option on Admin -> Plugins -> Text editors -> TinyMCE HTML editor -> General settings.
However, the only way to know what you can change is probably to look in https://github.com/moodle/moodle/blob/master/lib/editor/tinymce/lib.php#L119, and if you make a mistake here, you can probably break the editor completely.
To change take out the context menu, you probably need
{"plugins" => "lists,table,style,layer,advhr,advlink,emotions,inlinepopups,searchreplace,paste,directionality,fullscreen,nonbreaking,insertdatetime,save,iespell,preview,print,noneditable,visualchars,xhtmlxtras,template,pagebreak"}