remove unused imports from vendor.conf
remove unused imports from vendor.conf

```
2019/04/03 12:47:01 WARNING: package github.com/google/btree is unused, consider removing it from vendor.conf
2019/04/03 12:47:01 WARNING: package github.com/gregjones/httpcache is unused, consider removing it from vendor.conf
2019/04/03 12:47:01 WARNING: package github.com/peterbourgon/diskv is unused, consider removing it from vendor.conf
```

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

docker/cli,cpuguy83/cli,thaJeztah/cli,thaJeztah/cli,docker/cli,cpuguy83/cli