Revert "tf-m: Silence harmless version warning"

This reverts commit 33786b55.

Reverting it because the upstream PR has been abandoned, and
this extra CMake variable is causing the warning
"Manually-specified variables were not used by the project".

As of now the version warning doesn't come up.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>