Revert keeping zero bids in the auction state

[fizyk20]

This reverts the changes introduced in #4719, as they are no longer needed since #4728 got merged and they were the reason behind the recent issues in tests.

Since the logic introduced in #4719 expected zero bids to exist until the unbonds are processed, and 1.5 removed them upon bid withdrawal / undelegation, this caused issues during the upgrade: 2.0 was attempting to remove zero bids which had already been removed, and failing with a ValidatorNotFound error. Thus, the simplest fix was to revert those changes.

This PR also adds a few debug messages that proved to be useful in figuring out this issue.

Closes #4808, #4811, #4812