Remove recovery.signal at recovery end when both signal files are present.

Remove recovery.signal at recovery end when both signal files are present.

When both standby.signal and recovery.signal are present, standby.signal
takes precedence and the server runs in standby mode. Previously,
in this case, recovery.signal was not removed at the end of standby mode
(i.e., on promotion) or at the end of archive recovery, while standby.signal
was removed. As a result, a leftover recovery.signal could cause
a subsequent restart to enter archive recovery unexpectedly, potentially
preventing the server from starting. This behavior was surprising and
confusing to users.

This commit fixes the issue by updating the recovery code to remove
recovery.signal alongside standby.signal when both files are present and
recovery completes.

Because this code path is particularly sensitive and changes in recovery
behavior can be risky for stable branches, this change is applied only to
the master branch.

Reported-by: Nikolay Samokhvalov <nik@postgres.ai>
Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: David Steele <david@pgbackrest.org>
Discussion: https://postgr.es/m/CAM527d8PVAQFLt_ndTXE19F-XpDZui861882L0rLY3YihQB8qA@mail.gmail.com