Delete DatabaseMigrationStateSchedule (#2001)

We have been using it as a poor man's timed flag that triggers a system
behavior change after a certain time. We have no foreseeable future use
for it now that the DNS pull queue related code is deleted. If in the
future a need for such a flag arises, we are better off implementing a
proper flag system than hijacking this class any way.

db/src/main/resources/sql/schema/db-schema.sql.generated

@@ -241,12 +241,6 @@
         primary key (scope, type)
     );
 
-    create table "DatabaseMigrationStateSchedule" (
-       id int8 not null,
-        migration_transitions hstore,
-        primary key (id)
-    );
-
     create table "DelegationSignerData" (
        algorithm int4 not null,
         digest bytea not null,