hhvm: remove unnecessary upstart stanza, config option

The hhvm.server.graceful_shutdown_wait option will work only if the HHVM
server gets shut down by calling the /stop url on the admin server.The
only practical effect of hhvm.server.graceful_shutdown_wait in our setup
would be that:
- The socket communicating with apache will be reset immediately
- Requests will be processed internally until they either end or the
wait is reached; apache will still, ironically, get a 503.
- Any new connection will be refused, resulting in more 503s

So I think this option is basically useless with the FastCGI server,
unless the server is processing some operations that can be
unacknowledged.

Finally, if any kill signal is sent to HHVM, it will happily forget
about that setting and die instantly. Since upstart kills and
restarts/reloads its services by sending signals, this configuration has
zero effects on it.

Change-Id: If27c02833a54343ba3ea423f5a4185ff3608a4ef
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>