plot_utils: don't set the xlabel to Time by default

Leave the index (now correctly "Time") as xlabel by default.  Only set
it if explicitly asked for.
diff --git a/cr2/plot_utils.py b/cr2/plot_utils.py
index a9fa9da..46e1cc0 100644
--- a/cr2/plot_utils.py
+++ b/cr2/plot_utils.py
@@ -81,9 +81,8 @@
 
     """
 
-    if xlabel is None:
-        xlabel = "Time"
-    plt.xlabel(xlabel)
+    if xlabel is not None:
+        plt.xlabel(xlabel)
 
     if title:
         plt.title(title)