fix: mutable manager.remove_obsolete_receipts

[aasseman]

Remove unnecessary mutability that makes usage a little bit harder.

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Pull Request Test Coverage Report for Build 6696679791

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.176%

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Totals
Change from base Build 6696126630:	0.0%
Covered Lines:	1231
Relevant Lines:	1498

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💛 - Coveralls

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🤖 Cargo Audit Report 🤖

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[aasseman]

Rebased onto latest main

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