Delete databases from oracle 12c
The easiest and safest way to delete a database is using the RMAN DROP DATABASE command.

If on Linux, set up your ORACLE_HOME and ORACLE_SID environment variables appropriately then start the database(s) using startup mount exclusive restrict.  Then drop it using RMAN's DROP DATABASE.  If you want to delete the backups/contents of the FRA as well, then use DROP DATABASE INCLUDING BACKUPS.

You could also use the GUI (DBCA - Database Creation Assistant), but this may be trickier depending on the environment.