refactor: Remove unused defines in bitcoinunits.h

Summary:
This code was added in 7007402 and unused in the very next commit 2e4fee2

Backport of Core PR17869

Test Plan:

for IDENTIFIER in REAL_THIN_SP_HTML HAIR_SP_CP HAIR_SP_UTF8 HAIR_SP_HTML SIXPEREM_SP_CP SIXPEREM_SP_UTF8 SIXPEREM_SP_HTML FIGURE_SP_CP FIGURE_SP_UTF8 FIGURE_SP_HTML;
do
     grep -r $IDENTIFIER src/;
done

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7577