autotools build: Don't generate NMake snippets for introspection

We now have those parts directly in our NMake Makefiles used for
introspection, which results from the work to split out the source file
listings from gdk/Makefile.am and gtk/Makefile.am, so we could drop
those items from our autotools build files where those snippets were
generated during 'make dist'.
4 jobs for split-out-gdk-gtk-source-list-3-24 in 10 minutes and 10 seconds (queued for 2 seconds)
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