series/meta: remove the unused `index_names` attribute

We can leave the old ones in place.

README.md

@@ -189,7 +189,7 @@ You can see a series metadata:
 ```python
  >>> tsh.metadata(engine, 'my_series')
  {'tzaware': False, 'index_type': 'datetime64[ns]', 'value_type': 'float64',
- 'index_dtype': '<M8[ns]', 'index_names': [], 'value_dtype': '<f8'}
+ 'index_dtype': '<M8[ns]', 'value_dtype': '<f8'}
 ```
 
 We built a series with naive time stamps, but timezone-aware

test/test_tsio.py

@@ -334,7 +334,6 @@ def test_serie_metadata(engine, tsh):
     initialmeta = tsh.metadata(engine, 'ts-metadata')
     assert initialmeta == {
         'index_dtype': '<M8[ns]',
-        'index_names': [],
         'index_type': 'datetime64[ns]',
         'tzaware': False,
         'value_dtype': '<f8',
@@ -351,7 +350,6 @@ def test_serie_metadata(engine, tsh):
 
     assert tsh.metadata(engine, 'ts-metadata') == {
         'index_dtype': '<M8[ns]',
-        'index_names': [],
         'index_type': 'datetime64[ns]',
         'topic': 'banana spot price',
         'tzaware': False,
@@ -1081,7 +1079,6 @@ def test_serie_deletion(engine, tsh):
         'index_type': 'datetime64[ns]',
         'value_type': 'float64',
         'index_dtype': '<M8[ns]',
-        'index_names': [],
         'value_dtype': '<f8'
     }
 
@@ -1113,7 +1110,6 @@ def test_serie_deletion(engine, tsh):
         'index_type': 'datetime64[ns, UTC]',
         'value_type': 'float64',
         'index_dtype': '|M8[ns]',
-        'index_names': [],
         'value_dtype': '<f8'
     }
 

tshistory/tsio.py

@@ -32,7 +32,6 @@ class timeseries(SeriesServices):
     metakeys = {
         'tzaware',
         'index_type',
-        'index_names',
         'index_dtype',
         'value_dtype',
         'value_type'
@@ -662,11 +661,9 @@ class timeseries(SeriesServices):
 
     def _series_initial_meta(self, cn, name, ts):
         index = ts.index
-        inames = [name for name in index.names if name]
         return {
             'tzaware': tzaware_serie(ts),
             'index_type': index.dtype.name,
-            'index_names': inames,
             'index_dtype': index.dtype.str,
             'value_dtype': ts.dtypes.str,
             'value_type': ts.dtypes.name