docs: Remove (non-functional) JIRA README.md link

In 5364f90, the repo was updated to reference GitHub issues instead of a JIRA instance - but a reference to `[jira]` was missed and the link is now broken.

Removed reference.

Author: JackPGreen

README.md

@@ -68,7 +68,6 @@ Additional Resources
 --------------------
 
 + [Contributing patches](https://maven.apache.org/guides/development/guide-maven-development.html#Creating_and_submitting_a_patch)
-+ [Apache Maven Plugin Tools JIRA project page][jira]
 + [Contributor License Agreement][cla]
 + [General GitHub documentation](https://help.github.com/)
 + [GitHub pull request documentation](https://help.github.com/send-pull-requests/)