File tempo of wav files - how to disable/delete?

A while ago I recorded samples using Studio One 3.
(Via Digital-In from my Soundcard - a synthesizer was coected.)
Since the project BPM was 60.00, the wav files have a file tempo of 60.
Now I want to remove the 60.00.
When I mark all events, it's possible to set the BPM to NONE (either in Inspector or by right-clicking.)

Ok, but when I now export the samples (by right-click -> event -> export selection),
it seems that the file tempo is still there.
When I import the exported wav files, it says again 60.00, not NONE.

I downloaded some samples from the Inteet and imported then.
Their file tempo is NONE.

How can I set the file tempo of my samples to NONE??

P.S: I could not find any helpful information in the reference manual.

Thanks for any help