In order to remove the X-AspNet-Version HTTP Header we need to append enableVersionHeader="false" tag to the existing httpRuntime line inside the <system.web> element in the Web.config file:
<system.web>
        httpRuntime requestValidationMode="2.0" executionTimeout="3600" maxRequestLength="5120" enableVersionHeader="false" />
 <!- etc. ->
Please note this change must be made to the existing httpRuntime and NOT added as a new line in the web.config file.
Run Live HTTP Headers Firefox plugin to test.