Remove non-working no-unit for label variables.

Author: pp-mo

lib/iris/fileformats/_nc_load_rules/helpers.py

@@ -1209,9 +1209,6 @@ def get_attr_units(cf_var, attributes, capture_invalid=False):
         attributes["invalid_units"] = attr_units
         attr_units = UNKNOWN_UNIT_STRING
 
-    if np.issubdtype(cf_var.dtype, np.str_):
-        attr_units = NO_UNIT_STRING
-
     if any(
         hasattr(cf_var.cf_data, name)
         for name in ("flag_values", "flag_masks", "flag_meanings")