Delete missing database
You don’t need to delete the already missing database.The user is configured to use a none existing or deleted database as a default database for the application. First, use trace and check for which application the user is failed to login, then on the application, change the default database to one of the existing databases. Sadequl’s post  might help you for step by step guide to fix the issue, https://www.mssqltips.com/sqlservertip/2581/sql-server-error-18456-finding-the-missing-databases/