Remove moz.build files

[nicolas-guichard]

Bug 2048396 removed the sync of moz.build files when vendoring application-services into the Firefox tree.

As pointed out by https://phabricator.services.mozilla.com/D298757#10668401, keeping them here is potentially confusing and I ended up confused indeed, so let's remove them as suggested.

Verified that mach vendor third_party/application-services/moz.yaml -r 8859eb4f50085aef8cad25428e820cb113d4319a didn't remove any moz.build files downstream.

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