Makefile: remove redundant flags to $(RM)

$(RM) will always expand to 'rm -f' which means we don't need to ignore
errors or pass this flag again.
diff --git a/Makefile.am b/Makefile.am
index 31d5cbb..8e8bcc0 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -201,8 +201,8 @@
 testsuite_test_dependencies_CPPFLAGS = $(TESTSUITE_CPPFLAGS)
 
 testsuite-distclean:
-	-$(RM) -rf $(ROOTFS)
-	-$(RM) testsuite/stamp-rootfs
+	$(RM) -r $(ROOTFS)
+	$(RM) testsuite/stamp-rootfs
 
 DISTCLEAN_LOCAL_HOOKS += testsuite-distclean
 EXTRA_DIST += testsuite/rootfs-pristine