Remove pending_delivery column from the wp_lifterlms_webhooks db table

[eri-trabiccolo]

See:
#211

we don't use that column anymore.

[thomasplevy]

@eri-trabiccolo are you planning to work on this before the release of #211 or push this off into the future? As I said in #211 I don't think it's a huge deal if we push it off into the future as long as this issue exists. Up to you

[eri-trabiccolo]

I wanted to push this in the future because I think it requires changes to the core, so that the logic it's all in one place - there.
I assigned it to me so that you didn't feel you had to do it :D

[eri-trabiccolo]

@thomasplevy ^