Fix for Crash due to System.AccessViolationException when calling OleDBSearch from multiple threads. (#6052)

* Removing non thread safe member variables, as well as the check to ensure that they have been disposed.

* Removing 'ExecuteQuery_ShouldDisposeAllConnections_AfterFunctionCall'.

This call previously required the use of member variables that were not thread safe.   There is no reason to share this state across threads.  Arguably this test verifies an internal implementation detail and is not suitable for a unit test anyway.

src/modules/launcher/Plugins/Microsoft.Plugin.Indexer/Main.cs

@@ -253,7 +253,6 @@ namespace Microsoft.Plugin.Indexer
             {
                 if (disposing)
                 {
-                    _search.Dispose();
                 }
 
                 disposedValue = true;