doc: update readme (#130)

follow-up on https://github.com/ConduitIO/conduit-connector-postgres/pull/113/files#diff-2f3155aea603bde2c43d7e87f9fd14baf61f282ae3df1873fd70dd6456abc472L46

README.md

@@ -39,7 +39,6 @@ Example configuration for CDC features:
  "url": "url",
  "key": "key",
  "table": "records",
- "columns": "key,column1,column2,column3",
  "cdcMode": "logrepl",
  "logrepl.publicationName": "meroxademo",
  "logrepl.slotName": "meroxademo"
@@ -51,11 +50,6 @@ Example configuration for CDC features:
 If no `key` field is provided, then the connector will attempt to look up the primary key column of the table. If that
 can't be determined it will fail.
 
-## Columns
-
-If no column names are provided in the config, then the connector will assume that all columns in the table should be
-returned.
-
 ## Configuration Options
 
 | name | description | required | default |