rgw: bucket meta remove don't overwrite entry point first

Fixes: #6056

When removing a bucket metadata entry we first unlink the bucket
and then we remove the bucket entrypoint object. Originally
when unlinking the bucket we first overwrote the bucket entrypoint
entry marking it as 'unlinked'. However, this is not really needed
as we're just about to remove it. The original version triggered
a bug, as we needed to propagate the new header version first (which
we didn't do, so the subsequent bucket removal failed).

Reviewed-by: Greg Farnum <greg@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>