Top nav improvements

[mxdvl]

Here’s a series of step I think we should take to follow-up on https://github.com/guardian/dotcom-rendering/issues/6436 :

• Remove the headerTopNav switch
• Refactor the headerContent logic to only have two possible paths
• Reduce the header files to two:
  • rename headerTopNav.scala.html to header.scala.html
    • adjust this file so it reuses some of the patterns originally defined in header.scala.html
  • rename header.scala.html to headerSlim.scala.html and remove all the non-slim code paths

[shesah]

related to #9341 ?