Remove the schedule for a released version. (#1735)

Koalas `1.2.0` was released as scheduled.

Author: ueshin

docs/source/development/contributing.rst

@@ -152,8 +152,6 @@ The chart below is the expected release dates of minor releases.
 +------------+---------+
 | Date       | Version |
 +============+=========+
-| 8/28 2020  | 1.2.0   |
-+------------+---------+
 | 10/9 2020  | 1.3.0   |
 +------------+---------+
 | 11/20 2020 | 1.4.0   |