chore: Remove `MessageSignature` from multitest

[codecov[bot]]

Codecov Report

Merging #241 (595cf2b) into feat/generics_support (3d78b45) will increase coverage by 0.84%. The diff coverage is 97.88%.

@@                    Coverage Diff                    @@
##           feat/generics_support     #241      +/-   ##
=========================================================
+ Coverage                  87.83%   88.68%   +0.84%     
=========================================================
  Files                         25       25              
  Lines                       1595     1670      +75     
=========================================================
+ Hits                        1401     1481      +80     
+ Misses                       194      189       -5     

Files	Coverage Δ
sylvia-derive/src/multitest.rs	97.19% <100.00%> (+3.02%)	:arrow_up:
sylvia-derive/src/message.rs	85.78% <97.02%> (+2.22%)	:arrow_up:

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[hashedone]

What is this change purpose?

[jawoznia]

What is this change purpose?

This change is not connected directly to the feature but I found it troublesome at some points to work with the original design. I decided to smuggle this small refactor in this feature to ease the further development and to rely on it in future refactor.

Before merge to main I will squash this commit to not pollute commit history with chore.