Comment out db:test:load

It's not required because we run db:migrate earlier. Also,
db/schema.rb is slightly incomplete (it doesn't include the index
index_users_on_username because of a Rails limitation).

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
           PGUSER: postgres
           PGPASSWORD: postgres
 
-      - run: bundle exec rake db:test:load
+      #- run: bundle exec rake db:test:load # not required because script/install.bash runs db:migrate
       - run: bundle exec rspec
 
       - uses: coverallsapp/github-action@v2