If adM/Wikipedia remote settings suggestions are enabled and then later disabled, they stick around in memory, in AdmWikipedia.#suggestionsMap. The map should be cleared or set to null, here in enable().

Unfortunately a lot of the tests accidentally rely on the current behavior where suggestions are not cleared, and that's why fixing this isn't trivial.

This bug is spun out from bug 1831656, see discussion in https://phabricator.services.mozilla.com/D176779#inline-981608