app-layer: fix 'detection-only' keyword

If we follow the description in the yaml file, we should disable
parsing if 'detection-only' keyword is used.

I did not test the result but the logic was looking wrong to my eyes.

PR builds:
- PR build: https://buildbot.openinfosecfoundation.org/builders/regit/builds/25
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/regit-pcap/builds/23