Remove redundant boundscheck in getindex(::String, ::UnitRange) (#61656)

This is unnecessarily boundschecked, since the boundscheck block above
already checks for the validity of the index.

This closes https://github.com/JuliaLang/julia/issues/24841. The only
redundant boundscheck now is that in `unsafe_copyto`, but in
https://github.com/JuliaLang/julia/pull/57946 it was decided that double
boundschecking was preferred.