Delete unnecessary eltype() method

[JamesWrigley]

This already has a generic definition in Base: https://github.com/JuliaLang/julia/blob/release-1.13/base/abstractarray.jl#L245-L246

Fixes these invalidations seen on 1.13:

 inserting eltype(::StructUtils.Selectors.List{T}) where T @ StructUtils.Selectors ~/.julia/packages/StructUtils/019BB/src/selectors.jl:35 invalidated:
   backedges: 1: superseding eltype(x) @ Base abstractarray.jl:245 with MethodInstance for eltype(::Union{ForwardDiff.Dual{T, V, P}, AbstractArray{<:ForwardDiff.Dual{T, V, P}}} where {T, V, P}) (3 children)
              2: superseding eltype(x) @ Base abstractarray.jl:245 with MethodInstance for eltype(::Union{Number, AbstractArray}) (78 children)
              3: superseding eltype(x) @ Base abstractarray.jl:245 with MethodInstance for eltype(::AbstractArray{<:Complex}) (120 children)
              4: superseding eltype(x) @ Base abstractarray.jl:245 with MethodInstance for eltype(::AbstractArray) (497 children)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@8ede229). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #69   +/-   ##
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  Coverage        ?   84.63%           
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  Files           ?        6           
  Lines           ?      989           
  Branches        ?        0           
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  Hits            ?      837           
  Misses          ?      152           
  Partials        ?        0           

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