Remove blib2to3 grammar cache logging (#3193)

As error logs are emitted often (they happen when Black's cache
directory is created after blib2to3 tries to write its cache) and cause
issues to be filed by users who think Black isn't working correctly.

These errors are expected for now and aren't a cause for concern so
let's remove them to stop worrying users (and new issues from being
opened). We can improve the blib2to3 caching mechanism to write its
cache at the end of a successful command line invocation later.