Remove bad dependency proxy manifest records

Remove records with a content_type after reverting
some breaking behavior.

changelogs/unreleased/revert-pull-by-digest.yml

+---
+title: Remove dependency_proxy_manifests records with content_type to prevent Dependency
+  Proxy failures
+merge_request: 53506
+author:
+type: fixed

db/post_migrate/20210205174154_remove_bad_dependency_proxy_manifests.rb

+# frozen_string_literal: true
+
+class RemoveBadDependencyProxyManifests < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def up
+    # We run destroy on each record because we need the callback to remove
+    # the underlying files
+    DependencyProxy::Manifest.where.not(content_type: nil).destroy_all # rubocop:disable Cop/DestroyAll
+  end
+
+  def down
+    # no op
+  end
+end

db/schema_migrations/20210205174154

+483d1b4a24086fa57efe7f3b3fa872cf793352f80aba5c25614f07eafa2d30c5
\ No newline at end of file

spec/migrations/20210205174154_remove_bad_dependency_proxy_manifests_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20210205174154_remove_bad_dependency_proxy_manifests.rb')
+
+RSpec.describe RemoveBadDependencyProxyManifests, schema: 20210128140157 do
+  let_it_be(:namespaces) { table(:namespaces) }
+  let_it_be(:dependency_proxy_manifests) { table(:dependency_proxy_manifests) }
+  let_it_be(:group) { namespaces.create!(type: 'Group', name: 'test', path: 'test') }
+
+  let_it_be(:dependency_proxy_manifest_with_content_type) do
+    dependency_proxy_manifests.create!(group_id: group.id, file: 'foo', file_name: 'foo', digest: 'asdf1234', content_type: 'content-type' )
+  end
+
+  let_it_be(:dependency_proxy_manifest_without_content_type) do
+    dependency_proxy_manifests.create!(group_id: group.id, file: 'bar', file_name: 'bar', digest: 'fdsa6789')
+  end
+
+  it 'removes the dependency_proxy_manifests with a content_type', :aggregate_failures do
+    expect(dependency_proxy_manifest_with_content_type).to be_present
+    expect(dependency_proxy_manifest_without_content_type).to be_present
+
+    expect { migrate! }.to change { dependency_proxy_manifests.count }.from(2).to(1)
+
+    expect(dependency_proxy_manifests.where.not(content_type: nil)).to be_empty
+    expect(dependency_proxy_manifest_without_content_type.reload).to be_present
+  end
+end