Remove args check in filter_by_date

The args check is already done where filter_by_date is referenced:
https://github.com/nextstrain/augur/blob/945b49af50cc72c4343673491531ae630db40040/augur/filter.py#L609

This avoids needing to update this condition in two places.

augur/filter.py

@@ -319,9 +319,8 @@ def filter_by_date(metadata, date_column="date", min_date=None, max_date=None):
     """
     strains = set(metadata.index.values)
 
-    # Skip this filter if no valid min/max date is given or the date column does
-    # not exist.
-    if (not min_date and not max_date) or date_column not in metadata.columns:
+    # Skip this filter if the date column does not exist.
+    if date_column not in metadata.columns:
         return strains
 
     dates = get_numerical_dates(metadata, date_col=date_column, fmt="%Y-%m-%d")