chore: remove google-cloud-resource-settings which is no longer being updated (#13652)

`google-cloud-resource-settings` is not being updated. See the search
below which still shows "protobuf <6":


https://github.com/search?q=repo%3Agoogleapis%2Fgoogle-cloud-python+%22%3C6.0.0%22&type=code


https://github.com/googleapis/google-cloud-python/blob/3609c907cf4c03bf08418de150c84c32da9dae1e/packages/google-cloud-resource-settings/setup.py#L50

See
googleapis/googleapis@68b6376
and https://cloud.google.com/resource-manager/docs/deprecations for the
justification to remove `google-cloud-resource-settings`.

Fixes b/396600991

Author: parthea