build: remove git submodule handling from main makefile

The only remaining user of submodules at build time is roms/SLOF,
which is handled in pc-bios/s390-ccw/Makefile.  Remove the relevant
code from the main makefile.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
4 files changed