Removed .NET 4.0 workaround.

runtime/runtime.build

@@ -51,7 +51,6 @@
             </filterchain>
         </copy>
         <if test="${version::get-major(framework::get-version(framework::get-target-framework() )) == 4}">
-            <echo message="[assembly: System.Security.SecurityRules(System.Security.SecurityRuleSet.Level1)]" file="AssemblyInfo.cs" append="true" />
             <property name="defs" value="${defs};NET_4_0;CLASSGC" />
         </if>
         <if test="${first-pass}">