migrations: drop FK licenses_module_path_fkey

The licenses table currently has two FKs to modules.
licenses_module_path_fkey is no longer needed and is dropped.

For golang/go#39629

Change-Id: Ibf5a6400b1cf90e82748dcc53f45d23fdc29286d
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/265077
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

migrations/000034_drop_licenses_module_path_fkey.down.sql

@@ -0,0 +1,12 @@
+-- Copyright 2020 The Go Authors. All rights reserved.
+-- Use of this source code is governed by a BSD-style
+-- license that can be found in the LICENSE file.
+
+BEGIN;
+
+ALTER TABLE licenses
+    ADD CONSTRAINT licenses_module_path_fkey
+        FOREIGN KEY (module_path, version)
+        REFERENCES modules(module_path, version) ON DELETE CASCADE;
+
+END;

migrations/000034_drop_licenses_module_path_fkey.up.sql

@@ -0,0 +1,9 @@
+-- Copyright 2020 The Go Authors. All rights reserved.
+-- Use of this source code is governed by a BSD-style
+-- license that can be found in the LICENSE file.
+
+BEGIN;
+
+ALTER TABLE licenses DROP CONSTRAINT IF EXISTS licenses_module_path_fkey;
+
+END;