### Summary

`ansible-vault encrypt_string --help` shows the `--output` option but it remains without effect.

### Issue Type

Bug Report

### Component Name

ansible-vault

### Ansible Version

```console
$ ansible --version
ansible 2.9.21
  config file = /etc/ansible/ansible.cfg
  configured module search path = ['/home/elavarde/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python3.9/site-packages/ansible
  executable location = /usr/bin/ansible
  python version = 3.9.5 (default, May 14 2021, 00:00:00) [GCC 11.1.1 20210428 (Red Hat 11.1.1-1)]
```


### Configuration

```console
$ ansible-config dump --only-changed
[... empty ...]
```


### OS / Environment

Fedora 34

### Steps to Reproduce

```shell
ansible-vault encrypt_string --ask-vault-pass --name myvar --output vault.yml 'some secret'
```


### Expected Results

Variable and its encrypted content are written to file and not to stdout _or_ the option `--output` isn't shown in the help.

Making the `--output` version working would probably be a duplicate of issue #59590, so probably removing the `--output` option which doesn't work could be a simpler step. More important for now is that it's consistent.

### Actual Results

```console
- output file isn't created/edited
- variable and its content are output to stdout
```


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
options = super(VaultCLI, self).post_process_args(options)
        display.verbosity = options.verbosity
        if options.vault_ids:
            for vault_id in options.vault_ids:
                if u';' in vault_id:
                    raise AnsibleOptionsError("'%s' is not a valid vault id. The character ';' is not allowed in vault ids" % vault_id)
        if getattr(options, 'output_file', None) and len(options.args) > 1:
            raise AnsibleOptionsError("At most one input file may be used with the --output option")
        if options.action == 'encrypt_string':
            if '-' in options.args or not options.args or options.encrypt_string_stdin_name:
                self.encrypt_string_read_stdin = True
            
            if options.encrypt_string_prompt and self.encrypt_string_read_stdin:
                raise AnsibleOptionsError('The --prompt option is not supported if also reading input from stdin')
        return options
    def run(self):