Add `removeStake` function to staking collection

[joshuahannan]

Issue To Be Solved

Users need to be able to transfer their staking objects if needed, so they need a way to remove them from a collection.

Suggest A Solution

• Add a removeStake(nodeID: String, delegatorID: UInt32?): @AnyResource function that returns the specified staking object
• The object cannot be removed if there are any locked tokens used
• Subtract the total amount of tokens stored in the staking object from unlockedTokensUsed
• Add the addStake function to the public interface
• Create a transaction to remove a staking object and transfer it to another account's staking collection
• Add a test for transferring