Fix a -Wunused-variable in the locale fuzzer

Summary:
This is no longer needed after D8890.
Also turn on -Werror when building the fuzzer to prevent this from
happening again.

Test Plan:

ninja bitcoin-fuzzers

Check the warning is gone.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8908