Remove suppression operator on session_start()

This was originally added to stop PHP on Windows complaining that it
couldn't garbage collect old sessions, but that was for an old version
of PHP years ago and doesn't seem to be an issue any longer. We
really shouldn't be suppressing this. If session_start() threw a
warning that it couldn't write to the filesystem, then we wouldn't
know about it at all.
Merge pull request #3418 from halkyon/remove_supression

Remove @ suppression operator on session_start()