ZTS: Fix zfs_receive_004_neg

zfs recv of an incremental stream that already exists is ignored, with
a message like:

    receiving incremental stream of pool/fs@incsnap into pool/fs@incsnap
    snap testpool/testfs@incsnap already exists; ignoring

And the command exits successfully (exit code 0).

The zfs_receive_004_neg test is expecting that a this case will fail,
with nonzero exit code.

The fix is to remove this specific command from the test case. This
lets us check that the remaining commands do in fact fail.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10055