accel/kvm: Remove kvm_cpu_synchronize_state() stub

Since commit 57038a92bb ("cpus: extract out kvm-specific code
to accel/kvm") the kvm_cpu_synchronize_state() stub is not
necessary.

Fixes: e0715f6abc ("kvm: remove kvm specific functions from global includes")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Message-Id: <20250703173248.44995-22-philmd@linaro.org>

accel/stubs/kvm-stub.c

@@ -29,10 +29,6 @@ void kvm_flush_coalesced_mmio_buffer(void)
 {
 }
 
-void kvm_cpu_synchronize_state(CPUState *cpu)
-{
-}
-
 bool kvm_has_sync_mmu(void)
 {
     return false;