core: remove dead code from pre-deposed refactor
core: remove dead code from pre-deposed refactor

CBD used to "borrow" the last slot of of the Tainted list in the State,
but as of #1078 it uses its own list. This seems like just a block of
old code that didn't get cleaned up from the refactor. Deposed resources
get their own destroy nodes, so they don't have to mess with the main
destroy node.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk