[jdl] cleanup jdl support.

[mshima]

Overview of the feature request

Implement jdl generator with blueprint support:

• move file manipulation to jdl generator.
• move configuration merge to jdl generator.
• move validation to the related generator.
• return application configuration exactly as defined in the jdl file (don't apply default config).

Motivation for or Use Case

Those tasks are already been done at generator level, it's duplicated at jdl.

Related issues or PR

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[mshima]

@DanielFran bounty claimed https://opencollective.com/generator-jhipster/expenses/177002.

[DanielFran]

@mshima approved