#5780 Remove service_worker for firefox's manifest.json build process

[martgil]

This PR removes background.service_worker for Firefox's manifest.json build process to remove such warning for unknown property service_worker for Manifest V2.

close #5780

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Tests (delete all except exactly one):

• Does not need tests (refactor only, docs or internal changes)

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• [x] addresses the issue it closes (if any)
• [x] code is readable and understandable
• [x] is accompanied with tests, or tests are not needed
• [x] is free of vulnerabilities
• [x] is documented clearly and usefully, or doesn't need documentation

[martgil]

@sosnovsky Ready for review. thank you!

[martgil]

For testing, just load a debug Firefox build from this PR and notice the error message reported in #5780 will no longer occur.