md: remove test for duplicate device when setting slot number.

md: remove test for duplicate device when setting slot number.

When setting the slot number on a device in an active array we
currently check that the number is not already in use.
We then call into the personality's hot_add_disk function
which performs the same test and returns the same error.

Thus the common test is not needed.

As we will shortly be changing some personalities to allow duplicates
in some cases (to support hot-replace), the common test will become
inconvenient.

So remove the common test.

Signed-off-by: NeilBrown <neilb@suse.de>