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)