No longer necessary because select2 doesn't allow you to enter free text.

Even though the UI no longer allows invalid answers since it doesn't support free text,
it's still possible for another question to update a combobox with an invalid answer.

This validation is now available to both comboboxes and dropdowns.

Without this, web apps initially sets the value of an empty combobox to null,
but then the knockoout value binding sets it to undefined, which counts as a
change and triggers answer validation.

This undoes 36ac455 and 0c19009, again removing the validation code
because dropdowns don't allow invalid selections. What those commits
missed is that the combobox needed to be updated to use the index for
rawAnswer. Previously, the combobox was backed by a plain text input,
which meant rawAnswer was text and the entry would look up that text's
index. Now that there's a dropdown, both the validation and the lookups
are no longer necessary. The only logic dealing with text is receiveMessage.

Previously, if options changed, the minimal dropdown would select whatever
option was at the same index as the previous answer, and the combobox
would should an 'invalid choice' error.