Use serde provided serialization for atomics

[svenski123]

Use serde provided functions for serializing / deserializing atomics instead of the functions in serde_utils which are functionally equivalent.

Remove unused functions in serde_utils.

Note serde_utils.rs is left as an empty file for other PRs that will add to it.

[ryoqun]

horray for working on this quickly!

@svenski123 it seems that our ci doesn't like empty files...

I think this file can just completely go away and put snapshot versioned code in serde_snapshot.rs or similar to make the file name more appropriate in #9980.

[ryoqun]

Ah, also if you're bothered with rebasing in general. Please feel free to to any git rebase --squash on #9980. :)

[ryoqun]

Really thank you!

[codecov]

Codecov Report

Merging #10096 into master will increase coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10096   +/-   ##
=======================================
  Coverage    81.5%    81.5%           
=======================================
  Files         283      282    -1     
  Lines       65930    65904   -26     
=======================================
- Hits        53785    53765   -20     
+ Misses      12145    12139    -6