md: remove set but not used variable 'bi_rdev'

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

drivers/md/md.c: In function 'md_integrity_add_rdev':
drivers/md/md.c:2149:24: warning:
 variable 'bi_rdev' set but not used [-Wunused-but-set-variable]

It not used any more after commit
  1501efad ("md/raid: only permit hot-add of compatible integrity profiles")

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Shaohua Li <shli@fb.com>