Remove beta::hhvm

beta::hhvm provisions HHVM using Facebook's packages on Ubuntu Precise, which
is not a useful approximation of the setup we're projecting for production.
This will be re-done by having a proper, Trusty-based HHVM module that is
shared by both beta and prod.

Change-Id: Ia91d64c3fc6807fa4007aa909513d57c06334c15