chore(3p): Clean up unused third-party packages

Removes the following currently unused packages:

* terraform-gcp
* cpp.googleapis
* nixery

Change-Id: I02c38f0e6d23b1b9008706ca5a3b9db51cc9b689
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2650
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: glittershark <grfn@gws.fyi>

bin/__dispatch.sh

@@ -52,9 +52,6 @@ case "${TARGET_TOOL}" in
   stern)
     attr="third_party.stern"
     ;;
-  terraform)
-    attr="third_party.terraform-gcp"
-    ;;
   *)
     echo "The tool '${TARGET_TOOL}' is currently not installed in this repository."
     exit 1