Delete unnecessary lastindex() method

[JamesWrigley]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Fixes these invalidations when loaded through CurveFit.jl:

 inserting lastindex(A::RecursiveArrayTools.ArrayPartition) @ RecursiveArrayTools ~/.julia/packages/RecursiveArrayTools/otwou/src/array_partition.jl:283 invalidated:                                                                          
   backedges: 1: superseding lastindex(a::AbstractArray) @ Base abstractarray.jl:426 with MethodInstance for lastindex(::AbstractVector{UInt8}) (506 children)

Should not be necessary because of the Base implementation: https://github.com/JuliaLang/julia/blob/8f47b58ef511dd703e3eb9aeb37a668fe4403ca5/base/abstractarray.jl#L452

[JamesWrigley]

Test failure looks unrelated.