Drop partitions_dynamic schema if it exists

The original MR got reverted and deployed to GitLab.com. We ship another
migration here to clean this up.

changelogs/unreleased/ab-revert-schema-create.yml

+---
+title: Drop partitions_dynamic schema if it exists
+merge_request: 35426
+author:
+type: other

db/migrate/20200626130220_drop_partitions_dynamic_schema_if_exists.rb

+# frozen_string_literal: true
+
+class DropPartitionsDynamicSchemaIfExists < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def up
+    # This targets GitLab.com only - we deployed a migration to create this schema, but reverted the change
+    execute 'DROP SCHEMA IF EXISTS partitions_dynamic'
+  end
+
+  def down
+    # no-op
+  end
+end

db/structure.sql

@@ -23453,5 +23453,6 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200623185440
 20200624075411
 20200625045442
+20200626130220
 \.