Issue #7620 - Remove Nightly flag for external download manager

[liuche]

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[liuche]

These failures don't seem related to external download manager:
https://firefoxci.taskcluster-artifacts.net/DdqiIv06T_SlUOvqX-MhOQ/0/public/reports/test/testDebugUnitTest/index.html

and the pre-push hook didn't fail

[liuche]

Failing on UI test which seems completely unrelated: deleteMultipleSelectionTest
unit test: https://firefoxci.taskcluster-artifacts.net/U_X77hZdRLK3qn6WlMXCJw/0/public/reports/test/testDebugUnitTest/index.html

[codecov-commenter]

Codecov Report

Merging #14632 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master   #14632      +/-   ##
============================================
- Coverage     29.86%   29.86%   -0.01%     
- Complexity     1165     1167       +2     
============================================
  Files           445      445              
  Lines         18071    18071              
  Branches       2347     2347              
============================================
- Hits           5397     5396       -1     
- Misses        12297    12299       +2     
+ Partials        377      376       -1     

Impacted Files	Coverage Δ	Complexity Δ
...pp/src/main/java/org/mozilla/fenix/FeatureFlags.kt	100.00% <ø> (ø)	4.00 <0.00> (ø)
...mponents/searchengine/FenixSearchEngineProvider.kt	62.16% <0.00%> (-1.81%)	13.00% <0.00%> (ø%)
...ug/java/org/mozilla/fenix/DebugFenixApplication.kt	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
...in/java/org/mozilla/fenix/settings/SupportUtils.kt	74.00% <0.00%> (+4.00%)	13.00% <0.00%> (+2.00%)

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