[libc++][test] Drop unnecessary typename and template for layout_meow::mapping (#74571)

This is a pure syntax cleanup, I don't need it for running libc++'s
tests with MSVC's STL.

This is possible because std::layout_meow and layout_wrapping_integral<1729>
aren't dependent types. We only need typename and template when reaching
into a dependent type, like `typename OtherLayout::template mapping<E2>` or
`typename layout_wrapping_integral<Wraps>::template mapping<Extents>`.