Fix deprecated_in_future lint

[Philippe-Cholet]

I was wondering if we should remove some deprecated parts when I encountered the lint deprecated_in_future.

usize::MAX is stable since 1.43.0. Our MSRV is 1.43.1 so this is fine and the module std::usize and usize::max_value both say "Deprecating in a future version" so I remove those.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (6814180) 94.38% compared to head (c84709e) 94.30%.
Report is 8 commits behind head on master.

Files	Patch %	Lines
src/sources.rs	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #877      +/-   ##
==========================================
- Coverage   94.38%   94.30%   -0.09%     
==========================================
  Files          48       48              
  Lines        6665     6672       +7     
==========================================
+ Hits         6291     6292       +1     
- Misses        374      380       +6     

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