Remove unnecessary type bounds on derive

[michaelsproul]

Redo of this PR after I bastardised it:

• chore: remove derivative dep #34

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.50%. Comparing base (a751223) to head (3ac6f54).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ssz/src/bitfield.rs	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
- Coverage   77.76%   77.50%   -0.27%     
==========================================
  Files          12       12              
  Lines         715      720       +5     
==========================================
+ Hits          556      558       +2     
- Misses        159      162       +3     

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