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>
diff --git a/doc/index.rst b/doc/index.rst
index 85fb421..7a434a2 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -20,7 +20,7 @@
    ``https://www.zephyrproject.org/doc/<version>``. The following documentation
    versions are available:
 
-   `Zephyr 1.1.0`_ | `Zephyr 1.2.0`_ | `Zephyr 1.3.0`_ | `Zephyr 1.4.0`_ | `Zephyr 1.5.0`_ | `Zephyr 1.6.0`_ | `Zephyr 1.7.0`_
+   `Zephyr 1.5.0`_ | `Zephyr 1.6.0`_ | `Zephyr 1.7.0`_
 
 Sections
 ********
@@ -57,7 +57,3 @@
 .. _Zephyr 1.7.0: https://www.zephyrproject.org/doc/1.7.0/
 .. _Zephyr 1.6.0: https://www.zephyrproject.org/doc/1.6.0/
 .. _Zephyr 1.5.0: https://www.zephyrproject.org/doc/1.5.0/
-.. _Zephyr 1.4.0: https://www.zephyrproject.org/doc/1.4.0/
-.. _Zephyr 1.3.0: https://www.zephyrproject.org/doc/1.3.0/
-.. _Zephyr 1.2.0: https://www.zephyrproject.org/doc/1.2.0/
-.. _Zephyr 1.1.0: https://www.zephyrproject.org/doc/1.1.0/