Fix the bibliography DOI output when the hyperref package is not loaded.

The last attempt at adding hyperref support for DOIs worked, but it
broke things when the hyperref package isn't loaded. This commit fixes
it by conditionally defining a no-op \href in the absense of the
hyperref package.

Now that I understand what's going on a little bit better, I was also
able to eliminate some of the urlbst boilerplate.