Remove mentions of NewPipe Legacy

[triallax]

The now-unmaintained NewPipe Legacy is still referenced on the website, most prominently in the home page. Those references should be removed.

Edit: I just noticed that #299 exists, but I'd say this issue should be kept open so that other mentions are removed too if necessary.

[litetex]

I think this can be closed as I can no longer find any references in the code (despite Blog posts) while doing a quick search: https://github.com/search?q=repo%3ATeamNewPipe%2Fwebsite%20Legacy&type=code

Also #299 was merged a year ago.

[TobiGr]

We still provide the beta and legacy app icons: https://newpipe.net/press/logo/
Should be easy to fix