remove outdated terms of use from Unicode conversion files

We have some conversion mapping data files that are derived (reformatted into the ICU conversion data source file format) from files published by Unicode. They have a copy of the headers of those files including old terms of use. In 2015, Unicode removed those special terms of use from the headers of its files and just referred to the standard Unicode terms of use (which covers ICU anyway).

We should make equivalent changes in our derived ICU files. That will remove the old special terms of use from ICU.

Compare

• https://github.com/unicode-org/icu/blob/main/icu4c/source/data/mappings/iso-8859_10-1998.ucm

• https://github.com/unicode-org/icu/blob/main/icu4c/source/data/mappings/iso-8859_11-2001.ucm

• https://github.com/unicode-org/icu/blob/main/icu4c/source/data/mappings/iso-8859_14-1998.ucm

with

• https://www.unicode.org/Public/MAPPINGS/ISO8859/8859-10.TXT

• https://www.unicode.org/Public/MAPPINGS/ISO8859/8859-11.TXT

• https://www.unicode.org/Public/MAPPINGS/ISO8859/8859-14.TXT

(See the date and version history lines there.)

Optional: Consider also updating the files in the icu-data repo.