compiler_ffi: remove `uses_legacy_ni` since it is unused now

Signed-off-by: Jean Privat <jean@pryen.org>
ffi: move a lot of code and services from AModule to MModule

AModule is not the right class to do or store model related things.
Migrating these services simplify the code.

Signed-off-by: Jean Privat <jean@pryen.org>