iwyu: Do not export C++ headers in most cases

`IWYU pragma: export` enforces the transitive inclusion of the headers,
which undermines the purpose of IWYU.

The remained cases seem useful and could be considered separately:
- `<cassert>` in `util/check.h`
- `<filesystem>` in `util/fs.h`
- `<chrono>` in `util/time.h`

src/util/strencodings.h

@@ -20,8 +20,8 @@
 #include <cstdint>
 #include <limits>
 #include <optional>
-#include <string>      // IWYU pragma: export
-#include <string_view> // IWYU pragma: export
+#include <string>
+#include <string_view>
 #include <system_error>
 #include <type_traits>
 #include <vector>

src/util/string.h

@@ -12,8 +12,8 @@
 #include <cstring>
 #include <locale>
 #include <sstream>
-#include <string>      // IWYU pragma: export
-#include <string_view> // IWYU pragma: export
+#include <string>
+#include <string_view>
 #include <vector>
 
 namespace util {