springboot Remove plugin management for exec-maven-plugin

We currently provide plugin management for exec-maven-plugin and we specify the start class (if start-class is set). This can lead to issues if you're using the plugin with the standard command line property, see https://github.com/spring-projects/spring-boot/pull/20690

Retrospectively, providing plugin management for the exec plugin does not make much more sense anymore given spring-boot:run is the recommended way to start a Spring Boot app from Maven.