This gets sticky because the data files are registered in the Oracle instance. You cannot simply delete them.
A solution was found in this forum post:
https://stackoverflow.com/questions/35138581/how-can-i-delete-a-datafile-that-has-no-tablespace-without-accessing-the-filesys1. Create a new tablespace
2. Add the datafile to the tablespace
3. Drop the tablespace and its datafiles
For example:
CREATE TABLESPACE MYAPP
DATAFILE '/mnt/data/app/oracle/oradata/MYAPP_02.dat'
SIZE 100M AUTOEXTEND ON;
alter tablespace MYAPP add datafile '/mnt/data/app/oracle/oradata/MYAPP_01.dat';
drop tablespace MYAPP including contents and datafiles;