dead code: Remove dead option in HexStr conversion

Summary:
Problem:

• Nothing uses the fspaces argument to HexStr() besides unit tests. This argument results in extra complexity and a small performance decrease within the function for branch evalulation.

Solution:

• Remove unused fspaces option.

Backport of Core PR15573

Test Plan:
grep -r 'HexStr([^,]*,[^,]*,' src/
ninja all check-all

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7738