build: Remove the Autotools build

CI and downstream packagers have been using the Meson build for a while
now, and we checked that it's idempotent to the Autotools build.

Having two build systems in tree doesn't make maintaining and releasing
GTK any easier, even if it's the stable/frozen branch.

examples/application3/Makefile.am

@@ -1,32 +0,0 @@
-GTK_CFLAGS =	 				\
-	-I$(top_srcdir)				\
-	$(GTK_DEP_CFLAGS)
-
-GTK_LIBS = 					\
-        $(top_builddir)/gtk/libgtk-3.la 	\
-        $(top_builddir)/gdk/libgdk-3.la 	\
-        $(GTK_DEP_LIBS)
-
-
-noinst_PROGRAMS = exampleapp
-
-exampleapp_CFLAGS = $(GTK_CFLAGS)
-exampleapp_LDADD = $(GTK_LIBS)
-exampleapp_SOURCES = 				\
-	main.c 					\
-	exampleapp.c exampleapp.h 		\
-	exampleappwin.c exampleappwin.h
-nodist_exampleapp_SOURCES = resources.c
-
-BUILT_SOURCES = resources.c
-
-resources.c: exampleapp.gresource.xml window.ui
-	$(AM_V_GEN) $(GLIB_COMPILE_RESOURCES) $(srcdir)/exampleapp.gresource.xml \
-		--target=$@ --sourcedir=$(srcdir) --generate-source
-
-EXTRA_DIST = 					\
-	window.ui 				\
-	exampleapp.gresource.xml		\
-	meson.build
-
--include $(top_srcdir)/git.mk