Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

src/timer/SDL_timer.c

@@ -368,7 +368,7 @@ SDL_TimerID SDL_AddTimerNS(Uint64 interval, SDL_NSTimerCallback callback, void *
     return SDL_CreateTimer(interval, NULL, callback, userdata);
 }
 
-SDL_bool SDL_RemoveTimer(SDL_TimerID id)
+bool SDL_RemoveTimer(SDL_TimerID id)
 {
     SDL_TimerData *data = &SDL_timer_data;
     SDL_TimerMap *prev, *entry;
@@ -502,7 +502,7 @@ SDL_TimerID SDL_AddTimerNS(Uint64 interval, SDL_NSTimerCallback callback, void *
     return SDL_CreateTimer(interval, NULL, callback, userdata);
 }
 
-SDL_bool SDL_RemoveTimer(SDL_TimerID id)
+bool SDL_RemoveTimer(SDL_TimerID id)
 {
     SDL_TimerData *data = &SDL_timer_data;
     SDL_TimerMap *prev, *entry;