remove unused __futex function and source file

the direct syscall or various thin and mostly-inline wrappers around
it are used instead internally. at some point a public futex function
should be added, but it's not yet clear what the signature should be,
and in the mean time this file is not useful.

src/internal/futex.h

@@ -16,6 +16,4 @@
 
 #define FUTEX_CLOCK_REALTIME 256
 
-int __futex(volatile int *, int, int, void *);
-
 #endif

src/thread/__futex.c

-#include "futex.h"
-#include "syscall.h"
-
-int __futex(volatile int *addr, int op, int val, void *ts)
-{
-	return syscall(SYS_futex, addr, op, val, ts);
-}