Always drop the least preferred HB peer when adding a new one.

When a BIP152 HB-mode peer is in the least preferred position and
disconnects, they will not be by ForNode on the next loop. They
will continue to sit in that position and prevent deactivating
HB mode for peers that are still connected.

There is no reason for them to stay in the list if already gone,
so drop the first element unconditionally if there are too many.

Fixes issue #9163.