JIRA should not consider the JIRA FishEye Plugin to be required

Following up on the comments in JRA-22633, I think it's a mistake that the JIRA FishEye Plugin is considered a required plugin. This is not core functionality to JIRA and should be optional.

It has some known performance issues when there are large numbers of projects in JIRA and/or repos in FishEye that necessitate disabling it in those situations.