patch 8.1.0745: compiler warnings for signed/unsigned string

Problem:    Compiler warnings for signed/unsigned string.
Solution:   Remove type casts. (John Marriott)
diff --git a/src/mbyte.c b/src/mbyte.c
index 9c40072..66a3cc2 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -560,7 +560,7 @@
 	else if (GetLastError() == ERROR_INVALID_PARAMETER)
 	{
 codepage_invalid:
-	    return (char_u *)N_("E543: Not a valid codepage");
+	    return N_("E543: Not a valid codepage");
 	}
     }
 #endif