arm/arm64: Kill the ARM_MAX_MEMORY() macro

For 32-bit guests, the maximum memory size is represented by the define
ARM_LOMAP_MAX_MEMORY, which ARM_MAX_MEMORY() returns.

For 64-bit guests, the RAM size is checked against the maximum allowed
by KVM in kvm__get_vm_type().

There are no users left for the ARM_MAX_MEMORY() macro, remove it.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>