md: fix double free of io_acct_set bioset

Now io_acct_set is alloc and free in personality. Remove the codes that
free io_acct_set in md_free and md_stop.

Fixes: 0c031fd3

 (md: Move alloc/free acct bioset in to personality)
Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>