Remove link to the original repository

[piroor]

Describe the bug

This repository was started as a fork of https://github.com/Shinichi-Ohki/confirm-mail and today we are quite far from it, so we won't create pull requests to the original project from this repository anymore. However, new PR for this repository is always started as a PR to the original project because this repository is still linked to the original repository, and I easily create wrong PR for the original.

To Reproduce

1. Clone this repository to your PC.
2. Create a feature branch locally.
3. Push the feature branch to GitHub.
4. Go to https://github.com/FlexConfirmMail/Thunderbird and you'll see a button to create a new PR from the pushed feature branch.
5. Start to create a PR from the branch.
6. You'll see a form to create PR with a base branch https://github.com/Shinichi-Ohki/confirm-mail/tree/master .

Expected behavior

https://github.com/FlexConfirmMail/Thunderbird/tree/master is the base branch of a new PR.

Your Environment

Versions of Thunderbird and FlexConfirmMail are unrelated to this problem.

Your Configuration

Configuration of FlexConfirmMail is unrelated to this problem.

Additional context

From the "settings" page of this repository we can make this repository independent from the original repository, but it needs an administrator privilege.

[ashie]

I've requested it from https://support.github.com/contact?flow=detach_fork&subject=Detach%20Fork&tags=rr-forks via virtual assistant.