util: Remove unused itostr

Summary:
This is a backport of Core PR18449 [1/2]
https://github.com/bitcoin/bitcoin/pull/18449/commits/fac96fff624a3ab65209dcd3378efb6e6ab47a58

Test Plan:

ninja all check-all
grep -R itostr ..

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8890