docs: Remove <code> from <summary> entries due to RNG error
    
    commit '96e55048' caused make check failure for virschematest:
    
    1929) Checking ../docs/news.xml against ../news.rng                     ... libvirt: XML Util error : XML document failed to validate against schema: Unable to validate doc against /home/jferlan/git/libvirt.work/docs/schemas/../news.rng
    Datatype element summary has child elements
    Element summary failed to validate content
    Datatype element summary has child elements
    Element summary failed to validate content
    ^[[31m^[[1mFAILED^[[0m
    
    That's because <code> elements don't appear to be allowed in the schema.
    Rather than attempt to fix the schema, figured it was simpler to just
    remove them and let the schema fix happen later.