hw/i386/pc: Remove unneeded class attribute "kvmclock_enabled"

hw/i386/pc: Remove unneeded class attribute "kvmclock_enabled"

PCMachineClass introduces the attribute into the class hierarchy and sets it to
true. There is no sub class overriding the attribute. Commit 30d2a17b46e9
"hw/i386: Remove the deprecated machines 0.12 up to 0.15" removed the last
overrides of this attribute. The attribute is now unneeded and can be removed.

Fixes: 30d2a17b46e9 "hw/i386: Remove the deprecated machines 0.12 up to 0.15"
Cc: Thomas Huth <thuth@redhat.com>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Message-ID: <20240224135851.100361-5-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>