Per weak consensus on this topic, remove suggestion to set

HISTORICAL_MAKE_WORLD from the text that's output.  This was committed
against the previous consensus.  Leave the documentation in this file
as a compromose.  The HISTORICAL_MAKE_WORLD knob is intentionally
obfuscated and we only trust people smart enough to read the Makefile
to use it.  All others have no business using it due to its danger,
unless DESTDIR is set.

Dissentors: grog, obrien, trhodes

Makefile

@@ -206,9 +206,6 @@ world:
 	@echo "'Rebuilding world', for how to upgrade your system."
 	@echo "Define DESTDIR to where you want to install FreeBSD,"
 	@echo "including /, to override this warning and proceed as usual."
-	@echo "You may get the historical 'make world' behavior by defining"
-	@echo "HISTORICAL_MAKE_WORLD.  You should understand the implications"
-	@echo "before doing this."
 	@echo ""
 	@echo "Bailing out now..."
 	@false