Remove php.ini from HHVM cookbook

HHVM nightly Debian package now includes /etc/hhvm/php.ini, so that
`apt-get install hhvm-nightly` will hang waiting for the user input
trying to resolve conflict in this file.

This causes
https://github.com/travis-ci/travis-ci/issues/2812.

We fix this issue by removing php.ini. We assume that
phpenv will re-add this line to /etc/hhvm/php.ini.