Remove unused variable makefile_am_files

The `makefile_am_files` was part of the previous build system where
automake was used to build Makefiles. Since 9d9d39a0de
this is not used anymore and can be removed.

build/build.mk

@@ -29,7 +29,6 @@ all: $(STAMP) $(ALWAYS)
 	@$(MAKE) -s -f build/build2.mk
 
 generated_lists:
-	@echo makefile_am_files = Zend/Makefile.am TSRM/Makefile.am > $@
 	@echo config_m4_files = Zend/Zend.m4 TSRM/tsrm.m4 TSRM/threads.m4 \
 		Zend/acinclude.m4 ext/*/config*.m4 sapi/*/config.m4 >> $@