openvpn: T3834: verify() is not allowed to change anything on the system (#3851)

Commit e3c71af1466 ("remove secrets file if the tunnel is deleted and fix
opmode commands") added a code path into verify() which removed files on the
system if TOTP was not defined.

This commit moves the code path to the appropriate generate() function.

(cherry picked from commit 40c835992db9217f48e54dbbf15a7fbf1dcba482)

Co-authored-by: Christian Breunig <christian@breunig.cc>