Firefox 55 - how to remove every single search engine?

To hide "Search for ... with:" box from the bottom of address bar autocomplete list, go to about:config and set browser.urlbar.oneOffSearches to false.

As far as I know you can't remove all search engines or at least it makes no sense for you because you can fix you problem without removing all search engines.

How to remove the unwanted search entry in the drop-down menu:

1. Go to about:preferences#search
2. Uncheck Provide search suggestions
3. Under One-Click Search Engines remove the blue tick symbol for every search engine.

Now it should look like this:

4. Open about:config
5. Click on I accept the risk!
6. Search for keyword
7. Change the value of keyword.enabled to false by double-clicking on it

Now it should look like this:

8. Now your problem should be fixed

Note: Your Firefox will look a bit different than mine because I use Firefox Nightly.
Sorry for that.