Remove misleading FMT_USE_WINDOWS_H

include/fmt/os.h

@@ -132,15 +132,8 @@ class error_code {
 
   int get() const FMT_NOEXCEPT { return value_; }
 };
-// Define FMT_USE_WINDOWS_H to 0 to disable use of windows.h.
-// All the functionality that relies on it will be disabled too.
-#ifndef _WIN32
-#  define FMT_USE_WINDOWS_H 0
-#elif !defined(FMT_USE_WINDOWS_H)
-#  define FMT_USE_WINDOWS_H 1
-#endif
 
-#if FMT_USE_WINDOWS_H
+#ifdef _WIN32
 namespace internal {
 // A converter from UTF-16 to UTF-8.
 // It is only provided for Windows since other systems support UTF-8 natively.
@@ -210,7 +203,7 @@ class windows_error : public system_error {
 // Can be used to report errors from destructors.
 FMT_API void report_windows_error(int error_code,
                                   string_view message) FMT_NOEXCEPT;
-#endif
+#endif  // _WIN32
 
 // A buffered file.
 class buffered_file {