[Meta Toolbar] Remove feature flag for dynamic bottom toolbar

[sblatz]

We have now enabled dynamic bottom toolbar for nightly/debug builds. Filing this so we eventually enable it for all builds.

These three issues are the most critical to verify before landing this in a wider release:

• [Bug] Pages with embedded content sometimes scroll toolbar, sometimes doesn't (like Github) #10073 embedded content scrolling (less critical)
• [Bug] Sometimes the dynamic address bar at the bottom cannot appear #10045 Dynamic toolbar not appearing
• [Bug] Dynamic Nav Bar leaves white space on certain websites #8768 Large white space left at top/bottom
• [Bug] Can't scroll when the page is taking almost the entire screen #10477 Can't scroll almost full pages (affects error pages)
• When this is ready to land, please ping @mcomella for Validate performance of bottom dynamic nav bar feature #10157

Top Toolbar Issues

• Note these currently exist in release.

• [Bug] Dynamic top toolbar does not respect embedded content scroll events #11389 (dupe of white space at bottom of page [Bug] Dynamic top toolbar leaves white space on certain sites #11388)

cc @vesta0 to track this :)

┆Issue is synchronized with this Jira Task

[liuche]

I'll be cutting the 5.0 release on Monday 4/27 with the new AC release, @sblatz is this ready to be turned on? I believe @AmyYLee will meet and go over this with you next week to decide if we want it in the Fennec/Fenix build. In the meantime, if there aren't any large outstanding issues, @vesta0 wanted to get this onto Firefox Preview.

[sblatz]

As discussed on Slack, I think we should actually block this feature flag on these two issues (caused by the same underlying GV bug):

#10073
and
#10045

[liuche]

We decided that we'll remove the feature flag for 5.2 so we can get more testing on it, even though we really need #10477 to be fixed if we want to put this in The Release.

[liuche]

The remaining issue is not a release blocker.