Add two edit mode, "delete" and "delete_all".
what values are being deleted depends on <test> as documented.
if the target object is same to what is tested, matching value there
will be deleted. otherwise all of values in the object will be deleted.
so this would means both edit mode will not take any expressions.

e.g.

Given that the testing is always true here, the following rules:

  <match>
    <test name="foo" compare="eq">
      <string>bar</string>
    </test>
    <edit name="foo" mode="delete"/>
  </match>

will removes "bar" string from "foo" object. and:

  <match>
    <test name="foo" compare="eq">
      <string>foo</string>
    </test>
    <edit name="bar" mode="delete"/>
  </match>

will removes all of values in "bar" object.