Fix Javadoc generation and links for the site

- Removed `maven-jxr-plugin`
- Generating no Javadoc JARs (this was already the case)
- Removed Javadoc JARs from the assembly in `log4j-distribution`
- Generating Javadoc HTML only for `log4j-core` and `log4j-api`
- Copying generated Javadoc HTML to `target/site/javadoc/<artifactId>`
1 file changed