Non-JSON output after calling instantiate with --output-json

[HCastano]

If you call cargo contract instantiate with --output json there are a couple of
lines at the top of the output which a) aren't JSON and b) shouldn't be there.

# In the ink! repository
cargo contract build --manifest-path ./examples/flipper/Cargo.toml
cargo contract instantiate \
    --manifest-path ./examples/flipper/Cargo.toml \
    --suri //Alice --output-json \
    --constructor default \
    --salt $(date +%s) \
    --skip-confirm > /tmp/deployment-output.json

Results in the following output:

    Success! Gas required estimated at 290067161
{
  "contract": "5FemE98T9GNHMqaJbmmF4NtFzfugRmzJWqvjWUpSpEniufEp",
  "code_hash": "0xc076912767f384703fae885ce7c809182c29e767ed67164e1b24d82864d77dcb",
  "events": [
    {
      "pallet": "Balances",
      "name": "Withdraw",
      "fields": [ ... ]
    }]

    ...
}

The Success! ... line should be removed.

Tested with cargo-contract@3e24722.