compose box [nfc]: Cut a small comment redundant with type

Flow already knows that these are the only three possible values
for this variable: if you delete or munge one of the cases, you get
a type error at the `ensureUnreachable` call.

Thanks to Leslie Ngo for spotting this:
  #5510 (comment)

src/compose/ComposeBox.js

@@ -401,7 +401,6 @@ export default function ComposeBox(props: Props): Node {
     if (validationErrors.length > 0) {
       const msg = validationErrors
         .map(error => {
-          // 'upload-in-progress' | 'message-empty' | 'mandatory-topic-empty'
           switch (error) {
             case 'upload-in-progress':
               return _('Please wait for the upload to complete.');