The current arrangement with Neo4j does not allow us to know for sure
that the user intends to use `@Transactional` for reactive or imperative
operations.

The main reason for that is that the only trigger for the reactive
auto-configuration is the presence of reactor on the classpath given
that the Neo4j driver ships with both styles in the same jar. And
reactor can be on the classpath for a number of reasons that are
unrelated to imperative access (typically `WebClient`).

This commits removes the auto-configuration of the
ReactiveNeo4jAutoConfiguration and makes sure that if the user defines
one, the regular `PlatformTransactionManager` backs off. This is
a temporarily situation until we can provide a better signal that the
user intends to primarily use imperative repositories.

Closes gh-22940