Delete unused method FilesystemInterface::IsAbsent.

webrtc/rtc_base/unixfilesystem.h

Index: webrtc/rtc_base/unixfilesystem.h
diff --git a/webrtc/rtc_base/unixfilesystem.h b/webrtc/rtc_base/unixfilesystem.h
index 7270b83778be7df3df2d5552f82c71cdbbdb86b1..3c157cb17142e0607e1a30d0577217df45d15264 100644
--- a/webrtc/rtc_base/unixfilesystem.h
+++ b/webrtc/rtc_base/unixfilesystem.h
@@ -46,10 +46,6 @@ class UnixFilesystem : public FilesystemInterface {
   // Returns true of pathname represents an existing file
   bool IsFile(const Pathname& pathname) override;
 
-  // Returns true if pathname refers to no filesystem object, every parent
-  // directory either exists, or is also absent.
-  bool IsAbsent(const Pathname& pathname) override;
-
   std::string TempFilename(const Pathname& dir,
                            const std::string& prefix) override;