tests: bsim: bluetooth: ll: multiple_id: Allow unpair old bond

In the multiple_central.sh test, same Central bonds with Peripheral
multiple times. Because Peripheral uses different identity for the next
bond, Host by default sees the conflict with the old bond and rejects
the new one. For the test we don't care about security and can simply
replace the old bond.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>