[backport#19299] remove unused variables and add missing includes

Summary:
fa09ec83f3f23dacb807c6b6393cabf2a984e4ff

Remove unused variables

fa193c6b1b7da8f72a399bfddb1497655ce1685c

Add missing includes to fix compile errors

fs.cpp:35:17: error: no member named 'strerror' in namespace 'std'

return std::strerror(errno);
       ~~~~~^

fs.cpp:49:9: error: use of undeclared identifier 'close'

close(fd);
^

2 errors generated.i

This is a backport of core#19299

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9429