Remove code that's always #if'd out

$ rg 'if defined\(OS_NACL\)$'
$ rg 'if defined\(OS_FREEBSD\)$'
$ rg 'if defined\(OS_AIX\)$'
$ rg 'if defined\(OS_FUCHSIA\)$'
$ rg 'if defined\(OS_CHROMEOS\)$'
$ rg 'if defined\(OS_ANDROID\)$'

There's more that are in complex #ifs still.

Change-Id: I748e283c7c195493d64b4589303c90b8395a3413
Reviewed-on: https://gn-review.googlesource.com/1601
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>