qgrayraster: use a cast to void to mark a variable as unused

Do not use a self-assignment, as that may trigger compiler warnings.

Casting to void is also what Q_UNUSED is doing these days, but we can't
use it here because qgrayraster.c does not include qglobal.h.

Remove a 15-years old comment related to the self assignment.

Found by clazy.

Change-Id: I8cc92b7a1b0b5fb13824959740243e17606ec753
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/gui/painting/qgrayraster.c

@@ -167,11 +167,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 
-  /* This macro is used to indicate that a function parameter is unused. */
-  /* Its purpose is simply to reduce compiler warnings.  Note also that  */
-  /* simply defining it as `(void)x' doesn't avoid warnings with certain */
-  /* ANSI compilers (e.g. LCC).                                          */
-#define QT_FT_UNUSED( x )  (x) = (x)
+#define QT_FT_UNUSED( x )  (void) x
 
   /* Disable the tracing mechanism for simplicity -- developers can      */
   /* activate it easily by redefining these two macros.                  */