curl_trc: remove a bad assertion

curl_trc: remove a bad assertion

- Remove DEBUGASSERT that an internal handle must not have user
  private_data set before calling the user's debug callback.

This is a follow-up to 0dc40b2a. The user can distinguish their easy
handle from an internal easy handle by setting CURLOPT_PRIVATE on their
easy handle. I had wrongly assumed that meant the user couldn't then
set CURLOPT_PRIVATE on an internal handle as well.

Bug: https://github.com/curl/curl/pull/12060#issuecomment-1754594697
Reported-by: Daniel Stenberg
Closes https://github.com/curl/curl/pull/12104