[ADT] Remove deprecated `llvm::array_lengthof`

`llvm::array_lengthof` was marked deprecated a bit over two months ago in
5758c824.  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