migrations: drop modules.readme_* columns

The columns modules.readme_file_path and modules.readme_contents are
dropped, since they are no longer being used.

For golang/go#39629

Change-Id: I3829bdeb4eea9a5b8fb4a706641714a3aa8bfefc
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/265097
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>

migrations/000037_drop_modules_readme_columns.down.sql

@@ -0,0 +1,11 @@
+-- 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 modules
+    ADD COLUMN readme_file_path TEXT,
+    ADD COLUMN readme_contents TEXT;
+
+END;

migrations/000037_drop_modules_readme_columns.up.sql

@@ -0,0 +1,11 @@
+-- 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 modules
+    DROP COLUMN readme_file_path,
+    DROP COLUMN readme_contents;
+
+END;