Fixes errant print statement in LTIAuthBackend

This PR removes a debugging `print` statement from `LTIAuthBackend.authenticate()`

One of our LTI tools was raising a *500 Internal Server* while running in CGI mode on sites.fas.harvard.edu and the apache logs showed this error: `malformed header from script. Bad header=1407779241: index.py`. I traced it down to this print statement that just outputs the oauth timestamp. Since it looks like the timestamp is being logged anyway, I went ahead and removed the print statement.

Just thought I'd send this change upstream. Thoughts?