Remove unused FileOperationCallback

This seems not to be used anywhere

Bug: 1007686
Change-Id: I760479dc9d150246a8d9cc33fd568fb60dbcd3c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2280974Reviewed-by: Daichi Hirono <hirono@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Quang Minh Tuan Nguyen <nqmtuan@google.com>
Cr-Commit-Position: refs/heads/master@{#785279}

components/drive/file_errors.h

@@ -33,9 +33,6 @@ enum FileError {
   FILE_ERROR_SERVICE_UNAVAILABLE = -18,
 };
 
-// Used as callbacks for file operations.
-typedef base::Callback<void(FileError error)> FileOperationCallback;
-
 // Returns a string representation of FileError.
 std::string FileErrorToString(FileError error);