Delete unused method FilesystemInterface::IsAbsent.

BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/3012583002
Cr-Original-Commit-Position: refs/heads/master@{#19620}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 125e95eaf6c04faec20f85b7d7949f961fc12aea
5 files changed