more sbrk fixes

proc.c

@@ -500,8 +500,6 @@ wakeup(void *chan)
   release(&ptable.lock);
 }
 
-#if 0
-
 // Kill the process with the given pid.
 // Process won't exit until it returns
 // to user space (see trap in trap.c).
@@ -525,8 +523,6 @@ kill(int pid)
   return -1;
 }
 
-#endif
-
 // Copy to either a user address, or kernel address,
 // depending on usr_dst.
 // Returns 0 on success, -1 on error.