python socket_util: Use correct fatal_signal function on error path.

python socket_util: Use correct fatal_signal function on error path.

The correct function to call here is "remove_file_to_unlink".  That is,
since the file has already been unlinked there is no need to keep it on
the list of files to unlink.

However, "unlink_file_now" simplifies the code, so we might as well use
that.

Signed-off-by: Ben Pfaff <blp@nicira.com>