Move all the filter logic of cast & flutter

1. Move the post filtering logic from
   (cast|flutter)/(cast|flutter)-removed-resources.txt
   to filters/(cast|flutter).json
2. Reduce cast/icudtl.dat by 8K & flutter/icudtl.dat by 6.2K
   by removing unnecessary entries in
res_index.res which the real res files was previously removed by
bin/icupkg.
3. Simplified copy_data.sh and diff_data.sh
4. Remove the reference of flutter/flutter-removed-resources.txt from
   flutter/README.md

Bug: chromium:943348
Change-Id: Iedaad3a5c05098aa8ad67a9bf7165ef74ee816e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/1623634


Reviewed-by: Jungshik Shin <jshin@chromium.org>