refactor: Remove redundant c_str() calls in formatting
Summary:
Our formatter, tinyformat, *never* needs c_str() for strings.
Remove redundant c_str() calls for:
- strprintf
- LogPrintf
- tfm::format
This is a backport of Core PR17279
Test Plan:
ninja all check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D5908