doc: remove releases before 2.7 from index

Remove all old releases from the index.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/doc/conf.py b/doc/conf.py
index f59acb5..3fc1705 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -165,11 +165,6 @@
         ("3.1.0", "/3.1.0/"),
         ("3.0.0", "/3.0.0/"),
         ("2.7.4 (LTS)", "/2.7.4/"),
-        ("2.6.0", "/2.6.0/"),
-        ("2.5.0", "/2.5.0/"),
-        ("2.4.0", "/2.4.0/"),
-        ("2.3.0", "/2.3.0/"),
-        ("1.14.1", "/1.14.1/"),
     ),
     "display_vcs_link": True,
     "reference_links": {