hw/arm/raspi: Remove obsolete use of -smp to set the soc 'enabled-cpus'

hw/arm/raspi: Remove obsolete use of -smp to set the soc 'enabled-cpus'

Since we enabled parallel TCG code generation for softmmu (see
commit 3468b59 "tcg: enable multiple TCG contexts in softmmu")
and its subsequent fix (commit 72649619 "add .min_cpus and
.default_cpus fields to machine_class"), the raspi machines are
restricted to always use their 4 cores:

See in hw/arm/raspi2 (with BCM283X_NCPUS set to 4):

  222 static void raspi2_machine_init(MachineClass *mc)
  223 {
  224     mc->desc = "Raspberry Pi 2";
  230     mc->max_cpus = BCM283X_NCPUS;
  231     mc->min_cpus = BCM283X_NCPUS;
  232     mc->default_cpus = BCM283X_NCPUS;
  235 };
  236 DEFINE_MACHINE("raspi2", raspi2_machine_init)

We can no longer use the -smp option, as we get:

  $ qemu-system-arm -M raspi2 -smp 1
  qemu-system-arm: Invalid SMP CPUs 1. The min CPUs supported by machine 'raspi2' is 4

Since we can not set the TYPE_BCM283x SOC "enabled-cpus" with -smp,
remove the unuseful code.

We can achieve the same by using the '-global bcm2836.enabled-cpus=1'
option.

Reported-by: Laurent Bonnans <laurent.bonnans@here.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200120235159.18510-2-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>