Remove reflective RecordFieldDiscoverer

[zml2008]

Currently, the record field discoverer has two implementations: one which accesses Class methods reflectively (from the J14/J15 era), and one in a multi-release folder which directly calls the same methods. As the J14/J15 record implementations were only preview features, they are mostly unused in production environments. Additionally, 14 is already out of support, and 15 is soon to be out of support (March 2023 for Azul's distribution), so there should not be many remaining users.

What few users there are can delay upgrading Configurate until they've been able to update to at least Java 16.

We can remove the reflection-based implementation in favor of the directly compiled implementation that we already have, to reduce required maintenance effort for the project.