Move "atleast one signature" validation out of `SignaturesOf`

[SamHSmith]

I am retiring the invariant of SignaturesOf which states that the structure must contain at least one signature. I'm doing this because the new topology needed a way to clear the signatures. Since tx.rs is the only place this invariant is used the code that is enforcing the invariant should be moved out of SignatureOf and into tx.rs.

[mversic]

The invariant was removed here following this discussion.

Removing this invariant might have adverse effect on other parts of the codebase. As far as I can see this invariant is relied upon during deserialization of SignaturesOf. Considering recent changes made by @Erigara that enforce validity invariants more strictly during consensus this might not be relied on anymore

Changes such as these that potentially have non-local effects can introduce logical errors. A more strict auditing process should be mandatory in cases such as this

[mversic]

@mversic confirm that this was already tackled