deps: minor ICU fixes: maint docs/tool, downloader

- Docs used the word "copy" when it really meant a tool is needed.
- README-FULL-ICU.txt was generated in binary mode, but it's a
text file. This breaks on Python3 for maintaining ICU
- The ICU downloader was broken (also probably python3). It's
basically dead code since 1a25e901


landed (full icu in repo), unless someone deleted the deps/icu-small
directory from their repo.

Co-Authored-By: Christian Clauss <cclauss@me.com>
PR-URL: https://github.com/nodejs/node/pull/32347



Reviewed-By: Ujjwal Sharma <ryzokuken@disroot.org>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>