[puppet_master] Switch from InterfacePtr::Close(zx_status_t) to Unbind()

Close(zx_status_t) is being removed since epitaphs are only meant to be
sent from server to client.

Test: fx run-test puppet_master_unittests
Change-Id: Iecb582d425e23fed8ef4c6229967f18e7b259f3a
diff --git a/bin/sessionmgr/puppet_master/puppet_master_impl_unittest.cc b/bin/sessionmgr/puppet_master/puppet_master_impl_unittest.cc
index 5e1a963..43508d6 100644
--- a/bin/sessionmgr/puppet_master/puppet_master_impl_unittest.cc
+++ b/bin/sessionmgr/puppet_master/puppet_master_impl_unittest.cc
@@ -104,7 +104,7 @@
   story->Execute([&](fuchsia::modular::ExecuteResult r) {
     callback_called = true;
   });
-  story.Close(ZX_OK);
+  story.Unbind();
   RunLoopUntil([&]() { return executor_.execute_count() > 0; });
   EXPECT_FALSE(callback_called);
   EXPECT_EQ(1, executor_.execute_count());