doc: remove listing of old documentation archives

Per Anas, remove references to the pre-1.5 release documentation
Archived content is still accessible if you know where it is:
https://www.zephyrproject.org/doc/1.3.0/

  (for example)

Change-Id: Ia17c9ff04a76b86516f804794d6e3adb1cc2980d
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>