liblp: Remove the slot-suffix requirement in MetadataBuilder.

Normally MetadataBuilder will refuse to create non-suffixed partitions
on A/B devices. There are some scenarios where this doesn't make sense,
like when gsid needs to build metadata files for
system_gsi/userdata_gsi. It also doesn't make sense for the "scratch"
partition, so we added exceptions.

It turns out that metadata created by gsid cannot be re-imported by
MetadataBuilder, because there's no opportunity to set the "ignore" flag
in MetadataBuilder's constructor. Rather than plumbing a flag through, I
think we should just remove this error. It has too many exceptions
already and it doesn't really protect against anything. The motivation was
to avoid confusion in fastbootd on retrofit devices (where there are two
super partitions), but it's a pretty minor concern.

Bug: 134536978
Test: liblp_test gtest
Change-Id: I4629a3c46070c35bcce1017096338e72aa234371