MIPS: KVM: Fix a build warning about variable set but not used

After commit 411740f5 ("KVM: MIPS/MMU: Implement KVM_CAP_SYNC_MMU")
old_pte is no longer used in kvm_mips_map_page(). So remove it to fix a
build warning about variable set but not used:

   arch/mips/kvm/mmu.c: In function 'kvm_mips_map_page':
>> arch/mips/kvm/mmu.c:701:29: warning: variable 'old_pte' set but not used [-Wunused-but-set-variable]
     701 |         pte_t *ptep, entry, old_pte;
         |                             ^~~~~~~

Cc: stable@vger.kernel.org
Fixes: 411740f5

 ("KVM: MIPS/MMU: Implement KVM_CAP_SYNC_MMU")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310070530.aARZCSfh-lkp@intel.com/
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>