Magento2 – How to Remove Sign In and Create Account Links from Header

You can also remove some of the top links in your local.xml by referencing them and removing them by name.

<reference name="top.links">
   <remove name="wishlist_link"/>
   <remove name="checkout_cart_link"/>
</reference>

This only work if the top link was created with a name like this:

<reference name="top.links">
   <block type="wishlist/links" name="wishlist_link" />
   <action method="addLinkBlock"><blockName>wishlist_link</blockName></action>
</reference>

For the log in and log out, since they aren't called with a "name", you can't remove it this way.

You can remove it from code also from admin setting:

From your question ifconfig="ultimo/header/signup"

Go to theme setting header section there may be signup enable/disable setting.

Second way using code:

Go to your mobile theme local.xml

<default>
  <reference name="top.links"> 
    <action method="removeLinkByUrl">
      <url helper="customer/account/create" />
    </action> 
  </reference>
</default>