Remove references to previous path as it is no longer required

[karinathomasbbc]

Overall changes

Deletes all references to previousPath as it was required for Single Page Application, which has now been removed (see #10849 and search for previousPath - it is no longer set, and is therefore always null).

This simplifies our analytics code, especially when getting the referrer (which is used for ATI and Chartbeat Analytics)

Code changes

• Delete all references to previousPath
• Delete many references to origin as it was required in conjunction with previousPath to construct the referrer
• Tidy up tests

Testing

• PR Checks
• ATI Analytics continues to set referrer correctly
  • With network tab open, navigate from http://localhost:7080/mundo -> click America Latina topic -> http://localhost:7080/mundo/topics/c7zp57yyz25t & check the ATI request includes the correct value for ref
• Chartbeat
  • Canonical: virtualReferrer is null
  • AMP: virtualReferrer is the documentReferrer