Deactivate tooltip on tab by default.

Codepath is **not** removed by this.

docs/source/whatsnew/pr/tooltip-on-tab.rst

@@ -0,0 +1,4 b''
+ * In notebook, Showing tooltip on tab has been deactivated by default to avoid
+   conflict with completion, and will be completly removed in future version.
+   Shift-Tab could still be used to invoke tooltip when inside function
+   signature and/or on selection.

IPython/html/static/notebook/js/config.js

@@ -67,7 +67,6 b' var IPython = (function (IPython) {'
              'diff'         :{'reg':[/^diff/]}
             },
 
-        tooltip_on_tab : true,
         };
 
     // use the same method to merge user configuration