You will be looking a long time as there is no such NEC requirement.
If the retrofit kit is a listed kit and if that listed kit has directions requiring the ballast to be removed then you might be able to cite NEC 110.3(B).
What safety issue do you see with leaving them in place?
I do not think there is a code requirement to do so since the items would no no be energized.
110.3(B) if the instructions say to do so and it is part of the listing of the retrofit kits.
110.3(B) Installation and Use.
Listed or labeled equipment
shall be installed and used in accordance with any instruc-
tions included in the listing or labeling
410.6 Listing Required.
All luminaires, lampholders, and
retrofit kits shall be listed.
Edit: Merged threads and ended up just repeating what iwire said. Phooey.