multi: Remove index previous script snapshots.

[davecgh]

Now that the address index has been removed, there are no longer any indexers that require access to previous scripts. Therefore, this removes all of the code related to taking previous script snapshots and feeding them through to the indexers via the notifications.

It is split into several commits to ease the review process and ensure the code continues to build and pass all tests each step of the way.

The following is an overview of the changes:

• Removes PrevScripts field from the indexers.IndexNtfn index notification
• Removes indexers.indexNeedsInputs function
• Removes indexers.NeedsInputser interface
• Removes blockchain.PrevScripts field from blockchain.BlockConnectedNtfnsData and blockchain.BlockDisconnectedNtfnsData and associated code that takes the snapshots to populate those fields
• Removes code related to taking and housing previous script snapshots from blockchain
• Removes PrevScripts method from the indexers.ChainQueryer interface and associated implementations
• Removes blockchain.stxosToScriptSource function
• Removes indexers.PrevScripter interface