docs: remove xsltproc nonet flag

This seems to cause msys2-mingw32-meson build failure:

FAILED: docs/reference/gtk/gtk-query-immodules-3.0.1
"C:/msys64/mingw32/bin/xsltproc.EXE" "--nonet" "--stringparam" "man.output.quietly" "1" "--stringparam" "funcsynopsis.style" "ansi" "--stringparam" "man.th.extra1.suppress" "1" "--stringparam" "man.authors.section.enabled" "0" "--stringparam" "man.copyright.section.enabled" "0" "-o" "docs/reference/gtk/gtk-query-immodules-3.0.1" "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl" "../docs/reference/gtk/gtk-query-immodules-3.0.xml"
I/O error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl