build: i18n: reduce on-disk size

Reduces space by about 1MB with ICU 54 (over without this patch).
Also trims a few other source files, but only conditional on the exact ICU
version used. This is to future-proof - a file that is unneeded now may
be needed in future ICUs.
Merge commit 'bfdffa7' into srl-v0.12-IcuInside
build: i18n: rebuild icu-small after merge of joyent#8707