ion: remove obsolete stub

ion_mark_dangling_buffers_locked is no longer used anywhere. The main
implementation of the function was removed but the stub wasn't. Remove
the stub.

Change-Id: I8488c5dfdd93827c31ae41be849dfe0999a965d1
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>