Delete an oracle instance, the right way
You can delete databases with DBCA which takes care of most of it.

Or you can do as below, but this will do the same as removing the datafiles, redo logs, controlfiles manually.

sqlplus / as sysdba
startup mount exclusive restrict
exit
rman target /
drop database including backups noprompt;
exit


After this, you still have to remove the entry that belongs to the database from /etc/oratab, remove init.ora/spfile, password file from $ORACLE_HOME/dbs, and clean log directories (adump, bdump, cdump, udump).