I had an entire answer prepped, then realised this has changed in recent macOSes
Since High Sierra, the entire structure has been moved into what I can only call an unfathomable file.
I cannot make head or tail of it, however, if you throw it away it will clean your entire Login Items List.
I discovered this at https://discussions.apple.com/thread/8086931?answerId=32423169022#32423169022
There's a utility called Knock Knock which is very good for looking at these, but it doesn't contain any mechanism for changing them other than finding the reference in Finder. It also doesn't seem to find volumes to be mounted.
There's another app, Lingon which can show the list, including volumes, but again doesn't seem to want to modify it in any way [though that could be my unfamiliarity with the app]
So, I'd go for discarding the
backgrounditems.btm file [keep a copy just in case], then recreating your Login Items manually.
This is how it used to work before High Sierra, just for historical reference.
The Login Items list is store in a file in
~/Library/Preferences/com.apple.loginitems.plist This es simply a text file, so you can open it in TextEdit, or for a nicer presentation, perhaps easier to follow, something like BBEdit - free version is fine.
Just to be safe, Opt/drag a copy to the desktop, in case anything goes wrong.
Each item in there is contained between opening & closing delineators, with each separate login item between
</dict> tags, like html or many other text/xml file types.
So, in theory, you should be able to find the item in the file & delete it entirely, from the opening to the closing tag.
Like this for an example item. Note exactly where the selection is to & from, so once deleted the file doesn't break the tags at all…
Once deleted, save the file & reboot.