How do I delete a user from the master database when the user has been deleted from the Server Logins using the SSMS Object Explorer I am using an Azure SQL Database. I deleted some users from a Azure SQL database after I saw them mentioned in a Vulnerability Assessment report. Here's how I deleted them:

Log in to SSMS
Expand the Object Explorer tree
Expand the 'Security' folder
Expand the 'Logins' folder
Highlight the user
Right-click and select 'Delete'

I went back to the Vulnerability Assessment blade and ran a new 'Scan' but the users I deleted still show up in the list. The list included SQL code to show that my users still exist. I ran that code back in SSMS in the master database and confirmed my users still exist. Here's the relevant code:
SELECT * FROM sys.database_principals
When I run the following code, I get an error saying the user does not exist ('or you do not have permission' - but I am a server admin so I ruled that out):
DROP LOGIN &lt;username&gt;
Note: I already deleted the users from all databases on the server.
How do I get rid of these logins?
There's two concepts with security at play here, and in general how SQL Server works. There are Logins and Users, which the differences are discussed a little further in this StackOverflow answer. You've deleted the Logins which is server level, but the Users associated to those Logins still exist at the database level, which has its own Security node you can expand and then a Users node below it.
Whenever you map a Login to a database (to grant database permissions) it creates a User in that database.
Example: