hw/arm: Stop specifying integratorcp as the default board

Currently for both qemu-system-arm and qemu-system-aarch64
the default board model if the user doesn't specify one
is the 'integratorcp'. This is a totally arbitrary historical
accident since it was the first board to be modelled.
That board is now just one target among many for us, and
is a very poor choice of default:
 * it's an ancient board that is now only found in the
   junkpiles of longtime ARM/Linux hackers, if at all
 * it's an ARMv5 CPU, when most distros are now assuming
   ARMv7
 * it's pretty much unmaintained in QEMU
 * it doesn't even have versatilepb's advantage of
   supporting PCI

Making it or any other board the default serves only
to confuse people new to ARM who expect something more
like the x86 monoculture. Remove the is_default marker
from integratorcp, and don't set it for any other board,
to give users a nudge that they need to think about
which board they want a QEMU model of. (QEMU will produce
the admittedly slightly cryptic error "No machine found.")

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
