Remove unused interface `SignedStateInfo.java`

[poulok]

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👾 Description of the issue

There is an interface SignedStateInfo.java. This interface is implemented by a single class (currently named SignedState.java but this ticket will refactor it to SignableState.java) and is not referenced by any other classes.

Steps to reproduce

See the SignedStateInfo.java interface. Observe that is is never referenced except for a single implementing class.

Proposed Solution:

Copy the javadocs for the methods in SignedStateInfo.java into the single implementing class's overriding methods and delete the SignedStateInfo.java interface.

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