libbrillo: Remove WatchFileDescriptor. am: 295ef84cb2 am: 4b4f0480d0 am: 48220c6930 am: d4bf0e9fd2

Original change: https://android-review.googlesource.com/c/platform/external/libbrillo/+/1371782

Change-Id: I51adb9195f8eb0ef29a452c5079bda6f686c7a24