hw/i386/x86: Remove X86MachineClass::save_tsc_khz field

hw/i386/x86: Remove X86MachineClass::save_tsc_khz field

The X86MachineClass::save_tsc_khz boolean was only used
by the pc-q35-2.5 and pc-i440fx-2.5 machines, which got
removed. Remove it and simplify tsc_khz_needed().

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: <20250512083948.39294-11-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>