redo-if{change,create}: print a useful message if REDO isn't set.

Again, I forgot to make vars.py not crash if the variables aren't set, so we
can print a useful error message.  But this time I have the right solution:
vars.py will do the checking for itself, and abort with a nice message.

redo-ifchange.py

@@ -45,10 +45,6 @@ def should_build(t):
     return not state.isbuilt(t) and dirty_deps(t, depth = '')
 
 
-if not vars.TARGET:
-    err('redo-ifchange: error: must be run from inside a .do\n')
-    sys.exit(100)
-
 rv = 202
 try:
     try: