Remove superflous iostreams include from header files (#6901)

src/storage/storage.cpp

@@ -31,7 +31,6 @@
 #include <cstdint>
 
 #include <fstream>
-#include <iostream>
 #include <iterator>
 #include <new>
 #include <string>

src/storage/storage_config.cpp

@@ -3,6 +3,9 @@
 #include "util/exception_utils.hpp"
 #include <boost/algorithm/string/case_conv.hpp>
 
+#include <istream>
+#include <string>
+
 namespace osrm::storage
 {
 std::istream &operator>>(std::istream &in, FeatureDataset &datasets)