Devtools sets non-existent property spring.reactor.debug

[mhalbritter]

spring.reactor.debug doesn't exist anymore. Here is some old code using it, and this ticket renamed it.

The new property name is spring.reactor.debug-agent.enabled.

[mhalbritter]

Looks like the debug is now automatically enabled if a dependency is on the classpath. The spring.reactor.debug property is only an escape hatch for disabling. So i guess we can remove the property setting from devtools completely.

See #17128