daemon-util: downgrade log level on ECONNREFUSED and friends

daemon-util: downgrade log level on ECONNREFUSED and friends

This partially reverts 36c557f7d41441bbd98a8965348dfe8050fc9c98, which
introduced notify_remove_fd() that logs in LOG_DEBUG. However,
notify_remove_fd_warn() is still called other library functions, e.g.
notify_push_fd(), and produces warning message about the failure in
removing fd from fdstore on shutdown.

During shutdown process, we get the following logs:
```
systemd-udevd[370]: Failed to send notify message to '/run/systemd/notify': Connection refused
systemd-udevd[370]: Failed to remove file descriptor "config-serialization" from the store, ignoring: Connection refused
systemd-udevd[370]: Failed to send notify message to '/run/systemd/notify': Connection refused
systemd-udevd[370]: Failed to push serialization fd to service manager: Connection refused
```
Here, the 1st, 3rd, and 4th messages are in LOG_DEBUG, but the 2nd one
was in LOG_WARNING before this commit, and this makes it also in LOG_DEBUG.

Follow-up for 472404aca5357b7e65cdddf418342070b0ccd4d2.