Use `singer-io/target-csv` instead of our own fork

Our fork (https://gitlab.com/meltano/target-csv) of https://github.com/singer-io/target-csv was created to fix target-csv#1 (closed) (aka https://github.com/singer-io/target-csv/issues/14) without having to wait for singer-io to release a new version. Now that that has happened (https://github.com/singer-io/target-csv/releases/tag/v0.3.0), we can deprecate our fork and change the pip_url for target-csv back to git+https://github.com/singer-io/target-csv.git so users benefit from the latest changes, like a new destination_path setting.