build: i18n: cleanup and respond to review

Respond to review comments from @misterdjules:

* Remove the prepare-icu-source.sh mechanism for creating
  an embedded deps/icu-small as per
  7cb4aad7f3c4da2910146d8ff7750cb27ccb664f - it was not
  currently used and thus deadwood.

* Updated README.md to clarify some of the options:
  that 'none' is the default for --with-intl, and that
  --with-icu-source works with 'small-icu'

* clarified the test-intl.js test case

* cleaned up and added documentation in configure and in nodedownload.py