Bug 1616641 - let fennec-nightly tasks ride along vanilla fenix nightlies

[MihaiTabara]

We no longer need standalone fennec-nightly graphs. Hence we remove their graph definitions and let those tasks ride along the vanilla fenix nightlies.

Moreover, we want the fennec-nightly push-apk job to push to beta track directly, rather than alpha track so that Fenix folks have one less thing to take care of, now that we're 100% migrated on Fenix nightly. That change is tracked in mozilla-releng/scriptworker-scripts#148

[MihaiTabara]

Tested via taskgraph locally and the three additional tasks show up successfully in the fenix graph: build, signing and pushapk.

[codecov-io]

Codecov Report

Merging #8579 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #8579      +/-   ##
===========================================
+ Coverage     18.66%   18.7%   +0.03%     
  Complexity      464     464              
===========================================
  Files           319     319              
  Lines         12754   12754              
  Branches       1680    1680              
===========================================
+ Hits           2381    2386       +5     
+ Misses        10172   10166       -6     
- Partials        201     202       +1

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/org/mozilla/fenix/components/Search.kt	91.66% <0%> (ø)	2% <0%> (ø)	⬇️
...mponents/searchengine/FenixSearchEngineProvider.kt	56.52% <0%> (+5.43%)	10% <0%> (ø)	⬇️

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[JohanLorenzo]

Agreed to remove fennec-nightly and make the fennec/nightly branch obsolete. I saw on Slack @pocmo agrees, so 👍