remove alt GNUR_HOME feature (no longer works)

com.oracle.truffle.r.native/Makefile

@@ -27,13 +27,7 @@
 
 export TOPDIR = $(CURDIR)
 export R_VERSION = 3.1.3
-
-ifndef GNUR_HOME
 export GNUR_HOME = $(TOPDIR)/gnur/R-$(R_VERSION)
-export GNUR_HOME_ORIGIN = local
-else
-export GNUR_HOME_ORIGIN = environment
-endif
 
 all: 
 	$(MAKE) -C gnur

com.oracle.truffle.r.native/gnur/Makefile

@@ -27,20 +27,12 @@ all: makegnur
 	$(MAKE) -f Makefile.platform
 	$(MAKE) -f Makefile.libs
 
-ifeq  ($(GNUR_HOME_ORIGIN), environment)
-makegnur:
-else
 makegnur:
 	$(MAKE) -f Makefile.gnur
-endif
 
 clean: cleangnur
 	$(MAKE) -f Makefile.libs clean
 	$(MAKE) -f Makefile.platform clean
 
-ifeq  ($(GNUR_HOME_ORIGIN), environment)
-cleangnur:
-else
 cleangnur:
 	$(MAKE) -f Makefile.gnur clean
-endif