iommu/vt-d: Remove set but not used variable

iommu/vt-d: Remove set but not used variable

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/iommu/intel/iommu.c:5643:27: warning: variable 'last_pfn' set but not used [-Wunused-but-set-variable]
5643 |  unsigned long start_pfn, last_pfn;
     |                           ^~~~~~~~

This variable is never used, so remove it.

Fixes: 2a2b8eaa5b25 ("iommu: Handle freelists when using deferred flushing in iommu drivers")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20201127013308.1833610-1-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>