libweave: Add removing of weave::Command::Observer Not a problem yet, but this is a cleaner approach. BUG=none TEST='FEATURES=test emerge-gizmo buffet' Change-Id: I1889ee293a3e1db34aad14a7166b7ebbe32a8c10 Reviewed-on: https://chromium-review.googlesource.com/290161 Reviewed-by: Vitaly Buka <vitalybuka@chromium.org> Commit-Queue: Vitaly Buka <vitalybuka@chromium.org> Trybot-Ready: Vitaly Buka <vitalybuka@chromium.org> Tested-by: Vitaly Buka <vitalybuka@chromium.org>