Do not add JSR305 to default dependencies

The fact that JSR305 is on the classpath always has ill effects on immutables.org when combined with Java EE annotations.

The annotations clash on package name and both are scope=runtime. immutables.org will needlessly generate JSR305 because it sees them on the classpath - requiring dance around making the package not leak into bundle manifest.

Remove jsr305 from default dependencies while keeping it in dependencyManagement section. This way downstreams will not have it by default and can opt in if they need them.