remove gitutils (#1797)

Contributes to rapidsai/build-planning#224

In the new RAPIDS branching strategy (https://docs.rapids.ai/notices/rsn0047/), the default development branch is `main` and releases are finalized on branches named `release/{YY.MM}`.

Looking for lingering uses of the previous branching pattern (`branch-{YY.MM}`) today, I found `cpp/scripts/gitutils.py` here. It looks like it's not used for anything (`git grep gitutils`) and hasn't had a substantative change in a while (https://github.com/rapidsai/cuvs/commits/main/cpp/scripts/gitutils.py), so this proposes just removing it.

This same script was also recently removed from cuML and RAFT:

* rapidsai/cuml#7784
* rapidsai/raft#2946

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #1797

Author: jameslamb