Remove the sha1_hash from the package_metadata table

I'm thinking about adding more fields to this table, and the sha1_hash values
will make this tricker.

Therefore, remove the value, and adjust the existing code to cope. This commit
also adds a new test which coveres some of the changed functionality.

sqitch/verify/remove_package_metadata_sha1_hash.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:remove_package_metadata_sha1_hash on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;