[ADT] Remove deprecated `llvm::array_lengthof`
`llvm::array_lengthof` was marked deprecated a bit over two months ago in
5758c824dab4851fd061201483987b338c051417. Now, finally remove it. Since this
function was the only one in `STLArrayExtras.h`, get rid of the whole header.
D118342 (2022-01) moved the function to `STLArrayExtras.h`. This function should
have very few users now (MaskRay made some downstream inspection in
https://reviews.llvm.org/D133502#3777981). D133502 deprecated the function.
Differential Revision: https://reviews.llvm.org/D138374