Do not add epoll_sub (syscall wrappers) for epoll in cmake

Nowadays mostly all supported OS'es has this wrappers, and some of them (like
SmartOS) has wrappers but doesn't have __NR_epoll* defines for syscall numbers,
so just drop them (instead of adding yet another check int cmake like autotools
has, since this will break building in cross-compile environment).

Also one minor note, configure doesn't added epoll_sub.c either, since it check
epoll_create() in runtime.

And I tested it in SmartOS and it even works (`make verify`).

Fixes: libevent#463

CMakeLists.txt

@@ -682,7 +682,7 @@ if(EVENT__HAVE_DEVPOLL)
 endif()
 
 if(EVENT__HAVE_EPOLL)
-    list(APPEND SRC_CORE epoll_sub.c epoll.c)
+    list(APPEND SRC_CORE epoll.c)
 endif()
 
 if(EVENT__HAVE_EVENT_PORTS)