Merge: modelize_class: Remove an unnecessary pre-condition

`build_classes` calls itself recursively on imported modules. So, there is no point to specify that imported modules must be processed first.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>

Pull-Request: #2233
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc src/modelize/modelize_class.nit
Simple merge