Remove pattern match on calling exec.stop/1

There is no need to pattern match on this function call. We check to
confirm that the process was stopped correctly in the next line. With
the current pattern match, we cause an error when the PID is already
stopped.

Author: ethunk

lib/vector/agent.ex

@@ -89,7 +89,7 @@ defmodule Vector.Agent do
 
   def terminate(message, agent) when message in [:normal, :shutdown] do
     :ok = Logger.log(agent, :info, "vector: Vector is stopping.")
-    :ok = :exec.stop(agent.os_pid)
+    :exec.stop(agent.os_pid)
     do_confirm_exit(agent)
     do_flush_messages(agent)
     :ok = Logger.log(agent, :info, "vector: Vector has stopped.")