Make sure clean clm cleans up obj dir

When user runs "./case.build --clean clm" make sure that the
obj files are cleaned up (removed) correctly.

In #1140 the corresponding change for cleaning up
CLM objs differ because CESM supports building shared libs for
CLM 4.5 and 5.0 (hence a separate definition for the CLM obj
dir depending on the version). ACME always builds static libs.

[BFB]

config/acme/machines/Makefile

@@ -867,7 +867,7 @@ cleanrof:
 
 cleanlnd:
 	$(RM) -f $(LIBROOT)/liblnd.a
-	$(RM) -fr $(LNDOBJDIR)
+	$(RM) -fr $(EXEROOT)/lnd/obj
 
 cleancsmshare:
 	$(RM) -f $(CSMSHARELIB)