chore: Remove LatencyTest.java from repo (#4177)

* chore: Remove LatencyTest.java from repo

* chore: ignore removed test

---------

Co-authored-by: Diego Alonso Marquez Palacios <diegomarquezp@google.com>

Author: lqiu96

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

@@ -1053,4 +1053,8 @@
     <className>com/google/cloud/spanner/connection/Connection</className>
     <method>java.time.Duration getTransactionTimeout()</method>
   </difference>
+  <difference>
+    <differenceType>8001</differenceType>
+    <className>com/google/cloud/spanner/LatencyTest</className>
+  </difference>
 </differences>