This function was used internally, but is no longer used. Users should check
the value of the `DOCKER_CONTENT_TRUST` environment variable instead.
This method was deprecated in 11d40488dd,
which was included in the 28.5.0 release, and has no known external users,
so removing it for 29.0
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>