types: Remove unused assignment

clang-tidy warns about this harmless but unused assignment.

Change-Id: I5255dd6b49775c4515ceab753295d915a390f5c9
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/types.hpp b/types.hpp
index 3279256..082d588 100644
--- a/types.hpp
+++ b/types.hpp
@@ -72,7 +72,7 @@
 {
     void operator()(sd_event* event) const
     {
-        event = sd_event_unref(event);
+        sd_event_unref(event);
     }
 };
 using EventPtr = std::unique_ptr<sd_event, EventDeleter>;