hvf: Remove deprecated hv_vcpu_flush() calls

hvf: Remove deprecated hv_vcpu_flush() calls

When building on macOS 11 [*], we get:

  In file included from ../target/i386/hvf/hvf.c:59:
  ../target/i386/hvf/vmx.h:174:5: error: 'hv_vcpu_flush' is deprecated: first deprecated in macOS 11.0 - This API has no effect and always returns HV_UNSUPPORTED [-Werror,-Wdeprecated-declarations]
      hv_vcpu_flush(vcpu);
      ^
  /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Hypervisor.framework/Headers/hv.h:364:20: note: 'hv_vcpu_flush' has been explicitly marked deprecated here
  extern hv_return_t hv_vcpu_flush(hv_vcpuid_t vcpu)
                     ^

Since this call "has no effect", simply remove it ¯\_(ツ)_/¯

Not very useful deprecation doc:
https://developer.apple.com/documentation/hypervisor/1441386-hv_vcpu_flush

[*] Also 10.15 (Catalina):
    https://lore.kernel.org/qemu-devel/Yd3DmSqZ1SiJwd7P@roolebo.dev/

Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Tested-by: Roman Bolshakov <r.bolshakov@yadro.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>