Requires ICU 63 or above be used when building v8
1. Remove unneeded #include of icu header files
2. Remove code inside "#if U_ICU_VERSION_MAJOR_NUM < x"
block where x is 63 or smaller.



Bug: v8:8401 v8:5751
Change-Id: I908b0d7d174df53d4296580fe7150417322b0b21
Reviewed-on: https://chromium-review.googlesource.com/c/1314112Reviewed-by: Jungshik Shin <jshin@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57341}