v1.14: removes lazy-static thread-pool from sigverify-shreds (backport of #30787)

[mergify]

This is an automatic backport of pull request #30787 done by Mergify.
Cherry-pick of c6e7aaf has failed:

On branch mergify/bp/v1.14/pr-30787
Your branch is up to date with 'origin/v1.14'.

You are currently cherry-picking commit c6e7aaf96.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   ledger/benches/sigverify_shreds.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/sigverify_shreds.rs
	both modified:   ledger/src/sigverify_shreds.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[codecov]

Codecov Report

Merging #30827 (1cce709) into v1.14 (11a24f2) will decrease coverage by 0.1%.
The diff coverage is 84.3%.

@@            Coverage Diff            @@
##            v1.14   #30827     +/-   ##
=========================================
- Coverage    82.1%    82.0%   -0.1%     
=========================================
  Files         663      663             
  Lines      184933   184950     +17     
=========================================
- Hits       151837   151825     -12     
- Misses      33096    33125     +29