refs: "make mrproper" should clean out generated refs files

Doing a "make mrproper" should force the regeneration of the
default and user refs files. Otherwise these become stale because
kdevops appears to check if they exist, but not if they are newer
than the source yaml files.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: chucklever

scripts/build.Makefile

@@ -8,6 +8,8 @@ clean: $(clean-subdirs)
 
 PHONY += mrproper
 mrproper:
+	@$(Q)rm -rf $(CURDIR)/workflows/linux/refs/default/
+	@$(Q)rm -rf $(CURDIR)/workflows/linux/refs/user/
 	@$(Q)rm -rf $(CURDIR)/include/config/
 	@$(Q)rm -rf $(CURDIR)/include/generated/
 	@$(Q)rm -f .config