stg: filter out existing Patchwork-Id from patch before import

[kvalo]

If a patch already has a Patchwork-Id (for example I have done that multiple times when picking patches from the pending branch and resubmitting them) pwcli adds a second one in the bottom:

Signed-off-by: Alice <alice@example.com>
Patchwork-Id: 1111
Signed-off-by: Bob <bob@example.com>
Patchwork-Id: 2222

But when pwcli reads the id from pending branch it uses the first id, which is the incorrect one.

One possible solution is to remove all lines which we start with "Patchwork-Id:". Another possibility is to reject the patch. Don't know what's the best.