user-manual.conf: don't specify [listingblock]

This is the config file we use when we build the user manual with AsciiDoc. The comment at the top of this chunk that we're removing says the following: "unbreak" docbook-xsl v1.68 for manpages (sic!). v1.69 works with or without this. This comes from d19fbc3c17 ("Documentation: add git user's manual", 2007-01-07), where it looks like this conf file in general and this snippet in particular was copy-pasted from asciidoc.conf. This chunk is very similar to something we just got rid of for the manpages, and because this appears to be aimed at v1.68 -- which we no longer support for the manpages as of a few commits ago --, it's tempting to get rid of this. That reveals an interesting aspect of "works with or without this": it turns out it actually works /better/ without! Dropping this makes us render code snippets and shell listings using <screen> rather than <literallayout>, just like Asciidoctor does. In user-manual.pdf, this puts the contents into dimmed-background, easy-to-distinguish-from-the-surrounding-text boxes, as opposed to white-background (transparent) boxes. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>