Do not install gpMgmt/bin/README files

These files used to be installed to ${prefix}/bin/ directory, but as
they are not user documents we should not install them.
Reviewed-by: NBradford D. Boyle <bboyle@pivotal.io>
Reviewed-by: NBob Bao <bbao@pivotal.io>

gpMgmt/Makefile

@@ -128,6 +128,7 @@ install: generate_greenplum_path_file
 	rm -rf $(DESTDIR)$(prefix)/bin/src
 	rm -rf $(DESTDIR)$(prefix)/bin/gppylib
 	rm -rf $(DESTDIR)$(prefix)/bin/gpload_test
+	rm -rf $(DESTDIR)$(prefix)/bin/README*
 	find $(DESTDIR)$(prefix)/lib/python/gppylib -name test -type d | xargs rm -rf
 
 clean distclean: