Remove (some) code relating to Java versions below 8

[stephan-herrmann]

Since JDT/Core dropped support for Java versions below 8, the following queries in JavaModelUtil are now de-facto constants:

• is1d8OrHigher() -> always true
• is1d2OrHigher(), is1d4OrHigher(), is50OrHigher(), is1d7OrHigher() -> always false

I came here when checking if JDT/UI still "uses" org.eclipse.jdt.annotation_v1. In theory, the quickfix UnresolvedElementsBaseSubProcessor.addNullityAnnotationTypesProposals() would still try to locate this bundle in pre-Java projects, but since such projects are no longer supported, the version dependence in that method can be removed.