[libc++][test][NFC] Remove unused inclusions of <iostream> (#134776)

Some test files for flat container adaptors redundantly include
`<iostream>` and, surprisingly, `<cstddef>`. This patch removes the
redundant inclusions.

Inclusions of `<vector>` is also removed since a sane implementation is
expected to make instantiation of `flat_(multi)map<K, V>` or
`flat_(multi)set<K>` valid when only `<flat_map>` or `<flat_set>` is
included.

Author: frederick-vs-ja

libcxx/test/std/containers/container.adaptors/flat.map/flat.map.iterators/reverse_iterator.pass.cpp

@@ -19,16 +19,13 @@
 // const_reverse_iterator crend()   const noexcept;
 
 #include <cassert>
-#include <cstddef>
 #include <deque>
 #include <flat_map>
 #include <functional>
-#include <vector>
 
 #include <iterator>
 
 #include "test_macros.h"
-#include <iostream>
 
 int main(int, char**) {
   {

libcxx/test/std/containers/container.adaptors/flat.multimap/flat.multimap.iterators/reverse_iterator.pass.cpp

@@ -21,16 +21,13 @@
 // const_reverse_iterator crend()   const noexcept;
 
 #include <cassert>
-#include <cstddef>
 #include <deque>
 #include <flat_map>
 #include <functional>
-#include <vector>
 
 #include <iterator>
 
 #include "test_macros.h"
-#include <iostream>
 
 int main(int, char**) {
   {

libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/reverse_iterator.pass.cpp

@@ -19,16 +19,13 @@
 // const_reverse_iterator crend()   const noexcept;
 
 #include <cassert>
-#include <cstddef>
 #include <deque>
 #include <flat_set>
 #include <functional>
-#include <vector>
 
 #include <iterator>
 
 #include "test_macros.h"
-#include <iostream>
 
 void test() {
   {

libcxx/test/std/containers/container.adaptors/flat.set/flat.set.iterators/reverse_iterator.pass.cpp

@@ -19,16 +19,13 @@
 // const_reverse_iterator crend()   const noexcept;
 
 #include <cassert>
-#include <cstddef>
 #include <deque>
 #include <flat_set>
 #include <functional>
-#include <vector>
 
 #include <iterator>
 
 #include "test_macros.h"
-#include <iostream>
 
 void test() {
   {