target/ppc: Remove PPC_DEBUG_SPR left-over comment

The PPC_DEBUG_SPR definition was fully removed in commit
6113563982 ("target/ppc: Clean up _spr_register et al").

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20260219191955.83815-3-philmd@linaro.org>

target/ppc/cpu_init.c

@@ -52,7 +52,6 @@
 #endif
 
 #include "cpu_init.h"
-/* #define PPC_DEBUG_SPR */
 /* #define USE_APPLE_GDB */
 
 static const Property powerpc_cpu_properties[] = {