fixed audioroute code

[jaydom28]

Traceback (most recent call last):
  File "/home/justin/.pyenv/versions/lute/lib/python3.9/site-packages/flask/app.py", line 2190, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/justin/.pyenv/versions/lute/lib/python3.9/site-packages/flask/app.py", line 1486, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/justin/.pyenv/versions/lute/lib/python3.9/site-packages/flask/app.py", line 1484, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/justin/.pyenv/versions/lute/lib/python3.9/site-packages/flask/app.py", line 1469, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "/home/justin/Documents/Repos/lute-v3-khmer/lute/useraudio/routes.py", line 18, in stream
    b = Book.find(bookid)
AttributeError: type object 'Book' has no attribute 'find'

While testing the khmer branch, I was noticing none of my audio was loading and I was getting this traceback
I copied the logic in another file lute/bookmarks/routes.py and now it appears to work fine

    br = BookRepository(db.session)
    book = br.find(bookid)

[jzohrab]

Thanks @jaydom28 . I got rid of a bunch of static helpers as part of #503 -- strange that lint didn't pick this up, so good catch! I'm checking all other ones manually now as well.