md: remove redundant code that is no longer reachable

And earlier commit removed the error label to two statements that
are now never reachable.  Since this code is now dead code, remove it.

Detected by CoverityScan, CID#1462409 ("Structurally dead code")

Fixes: d5d885fd ("md: introduce new personality funciton start()")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Shaohua Li <shli@fb.com>