Revert "Add docker group to Buildkite docker plugin config (#2699)"

This reverts commit 72926b5d0b947b00d4e658710fe2d5ac76b0fc44.

buildkite/bazelci.py

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index c9945e9..bdb3e5c 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -3062,7 +3062,6 @@
         "plugins": {
             "docker#v3.8.0": {
                 "always-pull": True,
-                "additional-groups": ["docker"],
                 "environment": env,
                 "image": image,
                 "network": "host",