Revert "Merge branch 'doc-fix-db-gem-install-instructions' into 'master'"

This reverts commit 66d8ef1a, reversing changes made to 6a8359f3.

The --with option was added in Bundler 1.10 but in this case it shouldn't even be needed: I've tested the original command locally and the pg gem was installed, so I believe the user that opened the original MR ran the wrong command (i.e. sudo -u git -H bundle install --without development test postgres --deployment instead of sudo -u git -H bundle install --without development test mysql --deployment.

See discussion at https://gitlab.com/gitlab-org/gitlab-ce/commit/66d8ef1ad9d5952b7e7f026658f71aafc861950e for more context.