docs: Remove <code> from <summary> entries due to RNG error

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.