Fix OC storage property removal

[PVince81]

Whenever a non-existing property must be removed, we should ignore the
error.

Also note: PROPPATCH spec says that removing a non-existing property is ok: http://www.webdav.org/specs/rfc2518.html#ELEMENT_remove

[PVince81]

To test, run this twice after setting the properties in question:

curl -D - -u einstein:relativity -X PROPPATCH -H "Content-Type: text/xml" --data-binary "@proppatch-remove.xml" "http://localhost:9140/remote.php/webdav/test"

with the file proppatch-remove.xml:

<?xml version="1.0" encoding="utf-8"?>
<D:propertyupdate xmlns:D="DAV:">
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/alpha"/>
    </D:prop>
  </D:remove>
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/beta"/>
    </D:prop>
  </D:remove>
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/gamma"/>
    </D:prop>
  </D:remove>
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/delta"/>
    </D:prop>
  </D:remove>
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/epsilon"/>
    </D:prop>
  </D:remove>
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/zeta"/>
    </D:prop>
  </D:remove>
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/eta"/>
    </D:prop>
  </D:remove>
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/theta"/>
    </D:prop>
  </D:remove>
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/iota"/>
    </D:prop>
  </D:remove>
  <D:remove>
    <D:prop>
      <somename xmlns="http://example.com/kappa"/>
    </D:prop>
  </D:remove>
</D:propertyupdate>

Before the fix we got 500 Internal server error because "data is unavailble".
After the fix we get a 207 response.