[#3504] remove unused references to spec_config.h

Author: andrei-pavel

src/bin/dhcp6/spec_config.h.pre.in

@@ -12,9 +12,6 @@ EXTRA_DIST = process_messages.mes libprocess.dox
 
 CLEANFILES = *.gcno *.gcda
 
-# Generated by configure files
-DISTCLEANFILES = spec_config.h.pre
-
 lib_LTLIBRARIES = libkea-process.la
 libkea_process_la_SOURCES  = cb_ctl_base.h
 libkea_process_la_SOURCES += config_base.cc config_base.h