ctdb-tools: Drop separate parallel+verbose stdout/stderr filtering

ctdb-tools: Drop separate parallel+verbose stdout/stderr filtering

This has been broken for 10 years since commit
9616959bd6938e4c5c3713fe986c1e17cbdc574c, which introduced the
separate filtering.  This commit was missing a redirect of the output
of stderr_filter() to stderr.

Since nobody depends on the separate filtering (i.e. nobody reported a
bug), just return to combining stdout and stderr, and filtering them
together.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>