Remove modules from repo

[rmeissner]

Currently all modules are part of this repo, which requires to update all modules whenever there are compiler version changes for the core contract. Also this mixes the deployment/ version management of these. Therefore the module related code should be moved to https://github.com/gnosis/safe-modules

Edit: As these modules where only used as references to get devs started on creating modules we should just remove the modules (as they are not audited). For getting started we have a separate issue safe-global/safe-modules#25