Remove unused files.

libchrome contained unused files. This CL removes them.
- BUILD.gn
  Currently gn is not used. We're working on it (in ChromeOS build),
  but not a few files are dropped, so we need to revisit here later.
- ios, win specific files.
- base/i18n/. It only contains base_i18n_export.h, which is not used.
- base/trace_event/etw_manifest/. It is for win.

Bug: 73270448
Test: Built locally. Treehugger.
Change-Id: I5c59af648573f3b670f380e3263c456eec296af7
21 files changed