[managementgroups] Remove managementgroups extension

[jiasli]

Fix #585

management-group command group is already in the main CLI.

The doc for managementgroups is useless and self-contradictory: az account management-group create vs az managementgroups group new

This PR removes the whole managementgroups extension to prevent the doc from being generated.

[azuresdkci]

If this PR is for a new extension or change to an existing extension, use the following to try out the changes in this PR:

docker run -it microsoft/azure-cli:latest
export EXT=<NAME>
pip install --upgrade --target ~/.azure/cliextensions/$EXT "git+https://github.com/jiasli/azure-cli-extensions.git@remove-managementgroups#subdirectory=src/$EXT&egg=$EXT"

[yonzhan]

add to S165.

[Juliehzl]

@zhoxing-ms Please take a look for the change and remove deprecate information in azure cli after the change.