Get rid of functors for second order

[gdalle]

DI source

• Remove private functors Gradient, PushforwardFixedSeed and InnerDerivative, which were used in second order operators

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.53%. Comparing base (2411653) to head (f2f84d9).

Files with missing lines	Patch %	Lines
DifferentiationInterface/src/second_order/hvp.jl	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #460      +/-   ##
==========================================
+ Coverage   98.51%   98.53%   +0.01%     
==========================================
  Files         106      106              
  Lines        4250     4234      -16     
==========================================
- Hits         4187     4172      -15     
+ Misses         63       62       -1     

Flag	Coverage Δ
DI	98.69% <81.81%> (+0.02%)	⬆️
DIT	98.21% <ø> (ø)

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