cros: Remove some OS platform ifdefs from ash

Ash only builds on Chrome OS, so there's no need for OS_CHROMEOS and OS_WIN
checks.

BUG=666773
TEST=compiles

Review-Url: https://codereview.chromium.org/2618143002
Cr-Commit-Position: refs/heads/master@{#442318}
54 files changed