grub: there's no need to cap dom0 memory since we can balloon dom0

NB: this means we're defaulting to a 'developer friendly' environment
rather than a 'production' environment. Developers like to have
lots of memory available for compilers, while in production it's
better to dedicate it to running VMs.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>