Of course you can remove the SecureLine feature if you don’t [intend to] use it - it’s the same as with many other optional avast! features. Then the related files are removed from avast! installation folder.
On the other hand, if you don’t use SecureLine, then the openvpn.exe file is not used anyway (so even if it were vulnerable, the problem could never manifest, i.e. you’d be safe).