[Feature]: Remove unbonding pausing logic

[mpoke]

Summary

Remove the unbonding pausing logic necessary for Interchain Security (ICS) as future versions of ICS will not require it anymore.

Problem Definition

Remove unnecessary code and clean up the state.

Proposed Feature

The unbonding pausing logic was introduced through this PR. It enables an external module to pause unbonding operations (i.e., undelegations, redelegations, and validator unbondings) by calling the PutUnbondingOnHold method. The unbonding operation can be un-paused by calling the UnbondingCanComplete method. This was necessary for ICS to block the unlocking of stake until the consumer unbonding period elapses. The ICS team is currently working on removing this logic (as it affects the UX) and, thus, the unbonding pausing logic in SDK will no longer be needed.

Note that the AfterUnbondingInitiated hook should also be removed.