sink.ctf.fs: remove unreachable error cases

sink.ctf.fs: remove unreachable error cases

Just before this switch, we already check if `next_status` is negative
and bail out.  It's therefore not possible for the `ERROR` and
`MEMORY_ERROR` cases to be reached.

Change-Id: I6afa4d2f57e0b3574bbbef0e60c6dc5ebb746fb3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3906
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>