Remove legacy RSS Importer WP-CLI command

[MiguelAxcar]

Is your enhancement related to a problem? Please describe.

While working on #901 we found the dev only RSS importer CLI command uses simplexml_load_file() which is not allowed on VIP. This command is old and for devs only, and not a supported feature, so no reason for making it VIP compliant as it doesn't add value to the user.

Designs

No response

Describe alternatives you've considered

• Delete RSS Importer command
• Remove any related code, such as registration / wiring
• Search and remove any mentions for the command on the documentation

Code of Conduct

• I agree to follow this project's Code of Conduct