Remove outdated documentation from hibernate-gradle-plugin README

[kaden-sharpin]

JPA model generation was removed from the hibernate-gradle-plugin in #6632 . This removes said section in the README.

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[kaden-sharpin]

I can make a JIRA ticket if necessary. I don't have an account and CONTRIBUTING.md says:

For documentation contributions, mainly to respect the project code style, especially in regards to the use of tabs - as mentioned above, code style templates are available for both IntelliJ IDEA and Eclipse IDEs. Ideally, these contributions would also have a corresponding JIRA issue, although this is less necessary for documentation contributions.

Thanks