XIOS: remove `iodef.xml` entirely

[jwallwork23]

With changes from recent PRs (#555, #559, #574) we are now able to provide a minimal XML file for XIOS. However, the iodef.xml file used for the XIOS tests still includes syntax to allow standard output and error to be printed to files:

<?xml version="1.0"?>
<simulation>
  <context id="xios">
    <variable_definition>
      <variable_group id="parameters">
        <variable id="print_file" type="bool">true</variable>
      </variable_group>
    </variable_definition>
  </context>
</simulation>

We should wrap more of the API to allow this to happen within neXtSIM-DG itself.

[jwallwork23]

Decided in today's meeting that the small XML file above would be fine. Closing as not needed.