patch 8.1.0810: too many #ifdefs

Problem:    Too many #ifdefs.
Solution:   Graduate FEAT_MBYTE, part 4.
diff --git a/src/os_win32.h b/src/os_win32.h
index 37f6106..6c4a349 100644
--- a/src/os_win32.h
+++ b/src/os_win32.h
@@ -213,14 +213,7 @@
 #endif
 
 /* Enable common dialogs input unicode from IME if possible. */
-#ifdef FEAT_MBYTE
-# define pDispatchMessage DispatchMessageW
-# define pGetMessage GetMessageW
-# define pIsDialogMessage IsDialogMessageW
-# define pPeekMessage PeekMessageW
-#else
-# define pDispatchMessage DispatchMessage
-# define pGetMessage GetMessage
-# define pIsDialogMessage IsDialogMessage
-# define pPeekMessage PeekMessage
-#endif
+#define pDispatchMessage DispatchMessageW
+#define pGetMessage GetMessageW
+#define pIsDialogMessage IsDialogMessageW
+#define pPeekMessage PeekMessageW