That PIN requirement (or more precise, a lock-screen requirement) is enforced by Android.
Check here: https://android.stackexchange.com/questions/25491/certificate-install-without-mandatory-pin-lockscreen
.. and you can see some hacks with root access to get around it, where in most cases it doesn't work for all devices.
In short: Google has made it mandatory to put protection on your device before you can install client certificates (and the certificates are removed in most cases if you disable it again). No (good) way to disable that requirement.