Revert "change valid-from for old sans2d idf. re #8844"

This reverts commit b675bbfe.

Code/Mantid/instrument/SANS2D_Definition.xml

@@ -2,7 +2,7 @@
 <!-- For help on the notation used to specify an Instrument Definition File 
      see http://www.mantidproject.org/IDF -->
 <instrument xmlns="http://www.mantidproject.org/IDF/1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.mantidproject.org/IDF/1.0 Schema/IDFSchema.xsd" 
- name="SANS2D" valid-from   ="2014-02-02 00:00:00"
+ name="SANS2D" valid-from   ="1900-01-31 23:59:59"
                           valid-to     ="2100-01-31 23:59:59"
 		          last-modified="2012-03-04 00:00:00">