Do not set native image system property in NativeListener

[sdeleuze]

After an initial report by @odrotbohm where the hacking/aot branch of https://github.com/odrotbohm/spring-restbucks demonstrated that just adding the spring-native dependency was breaking Mockito tests, we found the root cause was NativeListener setting the org.graalvm.nativeimage.imagecode system property combined with recent versions of Mockito/ByteBuddy now supporting GraalVM native image and reacting to this property.

It was done initially to increase consistency between JVM AOT and native, but it is probably better to avoid this kind of trick, and just remove any specific behavior based on org.graalvm.nativeimage.imagecode in Spring Framework 6+.