Remove references to unsupported IE6

IE6 is no longer supported in target builds by GWT.
Remove the dead code, rename still live IE support to IE8.

Change-Id: I8d0f20ee09a687f2692e196d9f25cef928dc3bb4

gerrit-gwtexpui/src/main/java/com/google/gwtexpui/linker/server/UserAgentRule.java

@@ -67,9 +67,6 @@ public class UserAgentRule {
         if (v >= 8000) {
           return "ie8";
         }
-        if (v >= 6000) {
-          return "ie6";
-        }
       }
       return null;