Do not include AC_CONFIG_FILES in BUILT_SOURCES

BUILT_SOURCES is used to trigger early generation of files in the `make`
process to workaround dependency problems:

    https://www.gnu.org/software/automake/manual/html_node/Sources.html

AC_CONFIG_FILES are generated by `configure`, so they are already
present in the file system even before `make` is called. Furthermore,
AC_CONFIG_FILES already properly handle dist and clean targets, so no
need to add them to EXTRA_DIST or DISTCLEANFILES:

    https://www.gnu.org/software/automake/manual/html_node/Requirements.html

script/Makefile.am

@@ -42,10 +42,6 @@ EXTRA_DIST = \
 	ethercat.service.in \
 	ifup-eoe.sh
 
-BUILT_SOURCES = \
-	ethercatctl \
-	ethercat.service
-
 if HAVE_SYSTEMD
 systemdsystemunit_DATA = ethercat.service
 endif