Simplify code at a few places

Remove some duplicate checks,
and unnecessary errors.
> The default is to scan immediately when the annotator is constructed 

Yes, but it is hardly used in practice, because in most of the cases, first we need to load and initialize the plugins that ship the more advanced document access strategies (like d-t-m or pdf).

So that default might not make too much sense.

> If the user passes `dontScan` in the options, we can expect them to obey the contract that `loadAnnotations` shouldn't be called until a scan has been initiated.

There was never such contract. On the contrary; the code was build explicitly and intentionally in such a way that made it possible to call `loadAnnotations()` without caring about scanning at all. (`loadAnnotations()` was smart enough to do it when necessary.) Scanning is an internal detail of Annotator. (Or even lower levels.) The users should not need to care about it.

> Please, just remove this scan.

Done in https://github.com/hypothesis/annotator/commit/a3258fab8a32e7d66579f4246f1b56c3fcdd2b5c.