Nope, nope, nopity nope.
Fired for cause, not on-field performance. And certainly didn't leave of his own free will. The twitter barrage he sent out after being fired proved that. Never formally admitted, but the man was fired for cause.
We had fella who worked in the building at the time, told us all about the co-ed girlfriend, the complaints by Jancek's wife, the embarrassment to his son (who was on the team), the complaints by players...it was a mess. Butch told him to knock it off. A week later he brought her into the building again. And just a few days after that, in early January, he was fired.
EDIT: I didn't do a thorough search, but a quick check revealed these two threads from back then that mention the Jancek-girlfriend thing that got him fired.
The Brain vs The Chart
'17' TN ATH Tee Higgins (Clemson commit)
iirc, there was actually an entire thread on Jancek's firing, one that included the updates from the young man inside the building. And if memory serves, he started telling us about the problems _before_ Jancek was even fired. Had a lot of credibility, as I recall it. But that's five years ago, and my brain is old, so maybe I'm remembering it wrong, haha.