deps: delete deps/icu-small/source/io

The previous commit removed the dependency on the code in that
directory.  This commit removes the directory itself and shrinks
the source tarball by about 200 kB.

PR-URL: #13656
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

Author: bnoordhuis