How to remove encryption of HFS+ External Volume

I've got an external HFS+ formatted drive that I encrypted some time ago, but want to make it a network attached device, which requires that it not be encrypted. And I want to preserve its contents.

Under previous versions of the Macintosh operating system, simply cntl-clicking on the drive icon revealed a Decrypt contextual menu item. That seems to be gone. Moreover, I can't find anything in the man pages for the diskutil (Terminal) command that would decrypt HFS+ (only APFS).

Does anyone have a suggestion how I might proceed?