Revert "Don't omit bsd.compiler.mk for the nested delete-old in buildworld."

The original motivation (src.opts.mk needing bsd.comppiler.mk to
determine if CXX is supported) is no longer relevant now that CXX is
non-optional.

This reverts commit b9cb80883b.

Makefile.inc1

@@ -1024,7 +1024,7 @@ _worldtmp: .PHONY
 	${_+_}@if [ -e "${WORLDTMP}" ]; then \
 		echo ">>> Deleting stale files in build tree..."; \
 		cd ${.CURDIR}; env PATH=${TMPPATH:Q}:${PATH:Q} ${WMAKE:NPATH=*} \
-		    -DBATCH_DELETE_OLD_FILES delete-old \
+		    _NO_INCLUDE_COMPILERMK=t -DBATCH_DELETE_OLD_FILES delete-old \
 		    delete-old-libs >/dev/null; \
 	fi
 	rm -rf ${WORLDTMP}/legacy/usr/include