Do not remove PYLIB_SRC_EXT during make clean/distclean

Commit 8190ed40 removed lockfile from
mainUtils, but did not remove a reference to its source directory in the
make clean/distclean target. As a result, because LOCKFILE_DIR is no
longer defined, the make clean/distclean target removes the
PYLIB_SRC_EXT directory.

gpMgmt/bin/Makefile

@@ -193,7 +193,6 @@ installcheck: installcheck-bash
 	$(MAKE) -C gpload_test $@
 
 clean distclean:
-	rm -rf $(PYLIB_SRC_EXT)/$(LOCKFILE_DIR)
 	rm -rf $(PYLIB_SRC_EXT)/$(PYLINT_DIR)
 	rm -rf $(PYLIB_SRC_EXT)/$(LOGILAB_COMMON_DIR)
 	rm -rf $(PYLIB_SRC_EXT)/$(LOGILAB_ASTNG_DIR)