Fix crash after ping of an attachment deleted via mon$attachments

[MochalovAlexey]

This PR fixes a crash that a customer encountered after deleting an attachment from mon$attachments.

A later ping correctly reports that the connection has been shut down. The crash happens during cleanup, when YEntry tries to release an interface that has already been destroyed.

The next field in YAttachment and the nextRef field in YEntry hold references to the same interface. The manual release call drops one reference, and setting next to nullptr drops another, destroying the object. YEntry still holds the old pointer and tries to release it when its destructor runs.

The fix removes the extra manual release so that each RefPtr releases its own reference once.

[dyemanov]

The original code used detach() here, but IIRC that time detach() wasn't decrementing the reference counter. When its behaviour was changed, this piece of code wasn't adjusted properly. It also preserved the extra decrement when later changed from detach() to release().