nilrt.conf: stop pinning mono versions
nilrt.conf: stop pinning mono versions

We needed to pin mono only because its default preferred version was
3.x, which wasn't building well. The default version has been bumped
since then.

Signed-off-by: Richard Tollerton <67c688e7f7482ba5c4ebe3d70bc77594748182a6@ni.com>

ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt