hw/arm/raspi: Don't bother setting default_cpu_type

hw/arm/raspi: Don't bother setting default_cpu_type

In commit 210f47840dd62, we changed the bcm2836 SoC object to
always create a CPU of the correct type for that SoC model. This
makes the default_cpu_type settings in the MachineClass structs
for the raspi2 and raspi3 boards redundant. We didn't change
those at the time because it would have meant a temporary
regression in a corner case of error handling if the user
requested a non-existing CPU type. The -cpu parse handling
changes in 2278b93941d42c3 mean that it no longer implicitly
depends on default_cpu_type for this to work, so we can now
delete the redundant default_cpu_type fields.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180420155547.9497-1-peter.maydell@linaro.org