hw/i386/pc: Remove PCMachineClass::broken_reserved_end field

hw/i386/pc: Remove PCMachineClass::broken_reserved_end field

The PCMachineClass::broken_reserved_end field was only used
by the pc-q35-2.4 and pc-i440fx-2.4 machines, which got removed.
Remove it and simplify pc_memory_init().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <20250512083948.39294-3-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>