Blink does not need it any more. Drop UTF-32 converters and aliases.

Save 320 bytes in ICU data. icuuc.so will be slightly smaller, too.

BUG=chromium:417850
TEST=None

Change-Id: Ib4fecc3fcc905c974e68a652e7f4722bfd271129
Reviewed-on: https://chromium-review.googlesource.com/576872


Reviewed-by: Joshua Bell <jsbell@chromium.org>