dm table: Fix W=1 build warning when mempool_needs_integrity is unused

The mempool_needs_integrity is unused. This, in particular, prevents
kernel builds with Clang, `make W=1` and CONFIG_WERROR=y:

drivers/md/dm-table.c:1052:7: error: variable 'mempool_needs_integrity' set but not used [-Werror,-Wunused-but-set-variable]
 1052 |         bool mempool_needs_integrity = t->integrity_supported;
      |              ^

Fix this by removing the leftover.

Fixes: 105ca2a2 ("block: split struct bio_integrity_payload")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>