Revert "Add warning for incompatibility with AtCoder 202004"

[statiolake]

Now AtCoder is planning to update its judge environment (see https://docs.google.com/spreadsheets/d/1HXyOXt5bKwhKWXruzUvfMFHQtBxfZQ0047W7VVObnXI). Even if I enable this warning now, we'll soon have to revert the warning anyway.

Reverts statiolake/proconio-rs#39.