Systemctl - RHEL 7 - Is it possible to add ExecStartPre to delete file?

Question

I need to delete some files on a Linux RHEL 7.8 just after my server reboot every sunday.

I tried to add this line before the ExecStart: ExecStartPre=/usr/bin/rm -f /tmp/xx.txt but it doesn't work.

In fact the files (a size of a few kilobytes) are never delete without any error.

I added this line with vim command, may be systemctl edit --full name_service is better but I don't really now how it work.

May be there is a better way to do that than add it in a service file ?

Any help would be appreciated