[ADT] Remove deprecated `llvm::array_lengthof`

[jloser]

Summary

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.

[MaskRay]

D118342 (2022-01) moved the function to STLArrayExtras.h . This function should have very few users now (I made some downstream inspection in https://reviews.llvm.org/D133502#3777981). D133502 deprecated the function.

llvm/include/llvm/ADT/STLArrayExtras.h

-//===- llvm/ADT/STLArrayExtras.h - additions to <array> ---------*- C++ -*-===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-//
-// This file contains some templates that are useful if you are working with the
-// STL at all.
-//
-// No library is required when using these functions.
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef LLVM_ADT_STLARRAYEXTRAS_H
-#define LLVM_ADT_STLARRAYEXTRAS_H
-
-#include <cstddef>
-
-#include "llvm/Support/Compiler.h"
-
-namespace llvm {
-
-//===----------------------------------------------------------------------===//
-//     Extra additions for arrays
-//===----------------------------------------------------------------------===//
-
-/// Find the length of an array.
-template <class T, std::size_t N>
-LLVM_DEPRECATED("Use std::size instead.", "std::size")
-constexpr inline size_t array_lengthof(T (&)[N]) {
-  return N;
-}
-
-} // end namespace llvm
-
-#endif // LLVM_ADT_STLARRAYEXTRAS_H