Revert the previous patch. The <target-platform> tag is not supported

Revert the previous patch. The <target-platform> tag is not supported
anymore. Instead Frederic Bouvier suggests to add a <number> tag to
the axis definition which accepts values for <windows> and <unix>
for now but which can be extended later on:

<axis>
 <desc>Rudder</desc>
 <number>
  <unix>2</unix>
  <windows>3</windows>
 </number>
 <binding>
  <command>property-scale</command>
  <property>/controls/flight/rudder</property>
  <offset type="double">0.0</offset>
  <factor type="double">1.0</factor>
  <power type="double">2.0</power>
 </binding>
</axis>