Add support for deleting nodes with constants

[Pablete1234]

Straight up taken from #1181:

3.4.1 Support deleting attributes using the constant

A consideration was that constants maybe should support "deleting" the attribute, example:

<map>
  <name>Map</name>
  <variant id="tournament-edition">TE</variant>
  
  <constants>
    <constant id="size">32</constant>
    <constant id="overfill" delete="true"/>
  </constants>

  <if variant="tournament-edition">
    <constant id="size">8</constant>
    <constant id="overfill">8</constant>
  </if>

  <team id="red" max="${size}" max-overfill="${overfill}"/>
  <team id="blue" max="${size}" max-overfill="${overfill}"/>
</map>

In here we want the ranked version to have max=8, max-overfill=8, and the default to have max=32, and no max-overfill at all.
Being able to set the constant to "no value" (via delete="true" to signal that you clearly are doing this on purpose, it's not that you forgot the value), it would strip the whole attribute.