configure: remove --with-git-submodules=

Reuse --enable/--disable-download to control git submodules as well.
Adjust the error messages of git-submodule.sh to refer to the new
option.

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