Drop --experimental_remote_merkle_tree_cache from Gerrit pipeline (#2888)

`--experimental_remote_merkle_tree_cache` was removed in Bazel 9.0.0
(https://github.com/bazelbuild/bazel/commit/30ca50950fdaff032925efe64c2690a9f05e074d),
causing the `rbe_ubuntu2204` test step in the Gerrit pipeline (now on
Bazel 9.1.0) to fail:
https://buildkite.com/bazel-testing/gerrit/builds/5.