chore: delete CloseableMonitor

errorprone is removing support for the annotation this uses

Author: dpcollins-google

google-cloud-pubsublite/clirr-ignored-differences.xml

@@ -33,4 +33,9 @@
     <className>com/google/cloud/pubsublite/internal/**</className>
     <method>*</method>
   </difference>
+  <difference>
+    <differenceType>8001</differenceType>
+    <className>com/google/cloud/pubsublite/internal/**</className>
+    <method>*</method>
+  </difference>
 </differences>