Removing go.mod for the v3-release branch

As discussed in full length here (#17), we have run into issues that forces us to abandon go modules, at least for the `v3.x.x` releases. After this is merged in, we can release a `v3.2.1+incompatible` version, which contains a security fix. 

Afterwards, we will work on non-breaking quality of life fixes and then eventually run a `v4` version, which most likely will then support go modules and have a new SIV-style import path.