Refactor: libcib: Drop F_CIB_EXISTING

Unused internally. Also unused in Pacemaker-1.1.11. At one point it was
part of the CIB client diff notifications, so it would have been useful
only to IPC clients if any checked it. Now it's not used even for that.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>