devel-docs: fix autotools dist rules.

Since I removed some files and forgot to change these rules. Though I
actually wonder if this still makes sense to distribute all these files
within the tarball anymore. It made sense in the way software was
distributed 20 years ago, but nowadays people who want to develop would
clone the git repo not get a tarball. We'll see.

devel-docs/Makefile.am

@@ -7,7 +7,7 @@ SUBDIRS = \
 	performance-logs
 
 EXTRA_DIST = \
-	README				\
+	README.md			\
 	contexts.txt			\
 	debug-plug-ins.txt		\
 	exif-handling.txt		\
@@ -16,8 +16,6 @@ EXTRA_DIST = \
 	libtool-instructions.txt	\
 	parasites.txt			\
 	release-howto.txt		\
-	structure.xml			\
-	submitting-patches.txt		\
 	tagging.txt			\
 	ui-framework.txt		\
 	undo.txt			\