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       |