feat: remove unused files

[orisano]

remove unused files to reduce the size of the layer.
389MB -> 255MB

[tianon]

Are these directories documented somewhere as being safe to remove post-build? (so we can link back to something to note why we do so)

[orisano]

about: /usr/local/go/pkg/bootstrap
https://github.com/golang/go/blob/master/src/cmd/dist/buildtool.go#L121-L125

// Use $GOROOT/pkg/bootstrap as the bootstrap workspace root.
// We use a subdirectory of $GOROOT/pkg because that's the
// space within $GOROOT where we store all generated objects.
// We could use a temporary directory outside $GOROOT instead,
// but it is easier to debug on failure if the files are in a known location.

[orisano]

about: /usr/local/go/pkg/obj/go-build/
it is a cache directory.
https://groups.google.com/forum/#!topic/golang-dev/qfa3mHN4ZPA

please read this issue:
golang/go#23299
https://go-review.googlesource.com/c/build/+/82095

[yosifkit]

cmd/release: delete go/pkg/obj

There's no point to keeping this in releases.
It probably shouldn't be created at all, but since it is,
delete it.

https://go-review.googlesource.com/c/build/+/82095

😄