xprtrdma: FMR does not need list_del_init()

Clean up.

Commit 38f1932e

 ("xprtrdma: Remove FMRs from the unmap list
after unmapping") utilized list_del_init() to try to prevent some
list corruption. The corruption was actually caused by the reply
handler racing with a signal. Now that MR invalidation is properly
serialized, list_del_init() can safely be replaced.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>