Removing default input prompt number.

In a notebook setting being able to delete and add cells makes it
virtually impossible to correctly guess what the next input
prompt number should be. We now follow the convention that our
prompts look like "In [ ]:" before execution.

IPython/frontend/html/notebook/static/js/notebook.js

@@ -302,7 +302,8 b' var IPython = (function (IPython) {'
         // TODO: Bounds check for i
         var i = this.index_or_selected(index);
         var cell = new IPython.CodeCell(this);
-        cell.set_input_prompt(this.next_prompt_number);
+        // cell.set_input_prompt(this.next_prompt_number);
+        cell.set_input_prompt();
         this.next_prompt_number = this.next_prompt_number + 1;
         this.insert_cell_before(cell, i);
         this.select(this.find_cell_index(cell));
@@ -314,7 +315,8 b' var IPython = (function (IPython) {'
         // TODO: Bounds check for i
         var i = this.index_or_selected(index);
         var cell = new IPython.CodeCell(this);
-        cell.set_input_prompt(this.next_prompt_number);
+        // cell.set_input_prompt(this.next_prompt_number);
+        cell.set_input_prompt();
         this.next_prompt_number = this.next_prompt_number + 1;
         this.insert_cell_after(cell, i);
         this.select(this.find_cell_index(cell));