Remove CODE_OF_CONDUCT.md and use the organization-wide CODE OF CONDUCT file

[seisman]

Description of proposed changes

Currently, CODE_OF_CONDUCT.md only contains a link to the org-wide CODE OF CONDUCT file (https://github.com/GenericMappingTools/.github/blob/main/CODE_OF_CONDUCT.md). If a repository doesn't have a CODE_OF_CONDUCT.md file, the organization-wide default file will be used and displayed (https://github.blog/changelog/2019-02-21-organization-wide-community-health-files/).

To view how the org-wide CODE_OF_CONDUCT file works, you can visit the try-gmt repository:

This PR removes the local CODE_OF_CONDUCT.md file.

xref:

1. https://github.blog/changelog/2019-02-21-organization-wide-community-health-files/
2. https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/creating-a-default-community-health-file

[weiji14]

Looks ok. Checked that the Code of Conduct link on the website sidebar (e.g. https://www.pygmt.org/v0.12.0) points to https://github.com/GenericMappingTools/.github/blob/main/CODE_OF_CONDUCT.md too.