Fixing memory leak in TemporaryFileSystem

Fixes #1679

Added logic to remove the ShutdownHook that
TemporaryFileSystem was injecting for cleaning up
temporary files created by Selenium, after the cleanup
was completed.

Signed-off-by: Luke Inman-Semerau <luke.semerau@gmail.com>

java/client/src/org/openqa/selenium/io/TemporaryFilesystem.java

@@ -145,6 +145,10 @@ boolean shouldReap() {
   }
 
   public boolean deleteBaseDir() {
-    return baseDir.delete();
+    boolean wasDeleted = baseDir.delete();
+    if (! baseDir.exists()) {
+      Runtime.getRuntime().removeShutdownHook(shutdownHook);
+    }
+    return wasDeleted;
   }
 }