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Be concise, but don’t forget to provide context for your question, explaining what you need help with using specific examples. Screenshots and links encouraged.
However, note that if this menu is the source of the page that it links to, that page will also be deleted. If this menu is simply linking to an existing page that has its source from somewhere else, then you can delete freely.
Yes you’ll need to find the main page on the left where that menu lives and delete the specific menu button.
Then if you also want to delete the entire main login portal you can do that too. Without additional context it’s hard for me to advise exactly what to do it without risk of deleting something unintended.