[libc++][test][NFC] Remove unused inclusion of <iostream>

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

Author: frederick-vs-ja

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

@@ -28,7 +28,6 @@
 #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

@@ -30,7 +30,6 @@
 #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

@@ -28,7 +28,6 @@
 #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

@@ -28,7 +28,6 @@
 #include <iterator>
 
 #include "test_macros.h"
-#include <iostream>
 
 void test() {
   {