Remove old non-handoff content search UI code

The Search team is actively working on refactoring the search input so that it can operate in multiple contexts (including in-content, like within the newtab and about:privatebrowsing pages).

The existing handoff address bar, in the meantime, is something that we're hoping to shrink down and simplify, so that the newtab team can experiment with different embedding techniques.

The non-handoff capabilities for the content search UI have been disabled by default since 2021 with no UI exposed for re-enabling it, and according to our telemetry, less than 0.3 percent of searches use the non-handoff mode, so we feel pretty comfortable removing it at this point.

Hey dao / mbeier / mak, any objections if I start whittling this thing down? Or would that get in the way of the existing refactoring work?

And to be clear, this is means to a particular end - we're attempting to do 2 things in parallel:

1. The Search team is working on making the existing search experience in the browser chrome available in content, like on about:newtab.
2. The HNT team is working on making it easier for external teams (like Search) to embed things on about:newtab.

So the end goal will be to make it easier for the Search input on newtab to be evolved into something much more featureful than the current handoff. It may, however, mean that timing-wise, there is a period of time when only handoff is available on about:newtab while we prepare the embedding mechanism.

I don't think this would affect any of the ongoing refactoring, though rather than Eng approval you need Search Product approval, as it's likely some users (that 0,3% you found) will be vocal about this removal. Of course at a certain point in the future the search widget will return, but that will require some time to properly support a search field in the content process.

hey folks, @mconley and I were chatting about the non-hand off mode (i.e. setting the browser.newtabpage.activity-stream.improvesearch.handoffToAwesomebar to false pref)

It makes sense to me to not port it over to the Project Expo work. Some rationale:

• Not many people know about it - only real power users do (0.3% as y’all looked into it). There’s a chance that we annoy them but we need to make way for our New Tab search bar revamp work
• There doesn’t seem to be any alternatives - Dao mentioned the team doesn’t have a way to port this over into Expo work and if they spend more cycles thinking about that, it’s going to delay our MCAB milestones
• It sounds like this is just a preference that wasn’t meant to be left in, so this really doesn’t need to be possible to do especially when it’s pretty outdated in user experience and functionality

let me know if there are any concerns with anything I said - as long as Jim Thomas is good to go, I'm happy to disable this. I'm not sure if there's any comms considering that this isn't strictly supposed to be user facing but if it's needed, let me know

https://hg.mozilla.org/mozilla-central/rev/b505281fe032
https://hg.mozilla.org/mozilla-central/rev/6818dd16d8cf