Remove confusing "search in fediverse"

Requested at Camp Chatons

The current toggle "In the fediverse" fetches results from a centralized index. Users expect that "search in fediverse" retrieves more results than the two other options, but it is not the case. Results are just from a different source. There is no relevance in builiding within Mobilizon software a feature that queries a specific index. Therefore we should remove the toggle "In the fediverse". The search tool is a website of its own: https://search.joinmobilizon.org/.