Remove and out of stock products, fix API remove argument format

[unicoder88]

Hi!

Our customer reported an issue that sold out products show in Clerk. After code review I found following issues:

• controller product/index is_salable filter is silently ignored
• when product goes out of stock (purchase), the code should not skip it and remove this product from Clerk
• even further, remove API call didn't work as it returning error like Invalid syntax in argument: products. Turned out, API format changed.
  • as described in [documentation|https://docs.clerk.io/reference] - ?products=123,456 -- doesn't work
  • as called by the module - ?products[0]=123&products[1]=456 -- doesn't work
  • the only value that is accepted - ?products[]=123&products[]=456

Please review :)