Revert "obs-ffmpeg: Improve NVENC detection"

This reverts commit 94b5982216f253f4f4d355109a6dcb81f3a3b980.

Reverting this commit because it had some negative side effects, such as
adding 500 milliseconds to the startup time.  NVENC detection should
really be done through its proper API, and not via creating an encoder
on startup.