Remove unused controller code and old view

What does this MR do and why?

Remove unused controller code and old view, which isn't linked from anywhere.

Nowadays, snippets are either shown on:

• the dashboard ("Your work")
• the "Explore" section
• a user's profile

Screenshots or screen recordings

Before	After
	No special handling for the username param anymore. Redirects as it already did when username was not provided.

How to set up and validate locally

• Go to http://gdk.test:3000/-/snippets?username=root (or any other username)
  • On master, see this ancient page in all its glory.
  • On this branch, get redirected to the correct pages.
  • On both branches, without the username param, you get redirected, too.