The tests for `std::ranges::lazy_split_view` heavily use a wrapper class around
`std::string` because `std::string` was not `constexpr` until recently. Where
possible, remove the wrapper class and extra functionality no longer needed.
Remove `libcxx/test/std/ranges/range.adaptors/range.lazy.split/small_string.h`
and inline its one use remaining in
`libcxx/test/std/ranges/range.adaptors/range.lazy.split/general.pass.cpp`.
Differential Revision: https://reviews.llvm.org/D126663
Note that this class was called just `split_view` in the original One
Ranges Proposal and was renamed to `lazy_split_view` by
[P2210](https://wg21.link/p2210).
Co-authored-by: zoecarver <z.zoelec2@gmail.com>
Differential Revision: https://reviews.llvm.org/D107500
