KVM: MIPS: Remove a "set but not used" variable

This fix a build warning:

   arch/mips/kvm/vz.c: In function '_kvm_vz_restore_htimer':
>> arch/mips/kvm/vz.c:392:10: warning: variable 'freeze_time' set but not used [-Wunused-but-set-variable]
     392 |  ktime_t freeze_time;
         |          ^~~~~~~~~~~

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Message-Id: <20210406024911.2008046-1-chenhuacai@loongson.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>