simplify proc macro validation

[ralexstokes]

from the Oak Security audit:

SimpleSerialize trait implements the derive function, which performs some actions on the passed input. One of them is input.data validation, carried out at ssz-rs-derive/src/lib.rs:548.
This function, depending on whether the validation is successful, has the option of returning ValidationState::Validated, or executing plain panic! when it turns out that the data is incorrect.
Consequently, after this action, there is no technical possibility that the state of input.data will still be Unvalidated. This means that the validation performed in ssz-rs-derive/src/lib.rs:550-553 is redundant.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (e37de56) 75.29% compared to head (46b2b4a) 75.29%.

❗ Current head 46b2b4a differs from pull request most recent head 6f63574. Consider uploading reports for the commit 6f63574 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #85   +/-   ##
=======================================
  Coverage   75.29%   75.29%           
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  Files          18       18           
  Lines         858      858           
=======================================
  Hits          646      646           
  Misses        212      212           

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