Remove standalone library file in favor of PSR-4

We deprecated the standalone library file here: duosecurity@d64a910

Unfortunately we didn't have good insight into how this library was being used
so we couldn't outright remove it. Our thought process was first we deprecate
it and leave the standalone file around pointing to the PSR-4 library file.
Then, if there were no complaints, we would remove the standalone file at a
later date. I think it's been long enough, and we haven't heard of anyone
having troubles with installation.