Generates <executeGoal> / <executePhase> from @goal and @phase annotations, which is incorrect

When writing a Scala mojo, if no `@executeGoal` or `@executePhase` annotation is present on the mojo, the plugin descriptor generated has `<executeGoal>` and `<executePhase>` elements anyway, taken from the `@goal` and `@phase` annotations. This is wrong; those elements should be generated from `@executeGoal` and `@executePhase` annotations only.

This change fixes it.