Drop support for `routine` versus `pod` building strategy

The necessity to support the `routine` building strategy is a kind of a limitation as it requires the operator to be able to manage the operations concerns and the build concern, delegating this to some external Maven processes but on the same running Pod. I think that we should remove the support of `routine` entirely, letting the operator to spin off a Pod and perform the build accordingly as it already happens with `pod` strategy. This would simplify the design and maintenance of the project and likely the usage of the operator with one less concern to care. Anyway it would be something to consider in a major release only. Feedback are welcome.

The issue names no files or tests. Start by tracing the `routine` and `pod` building-strategy definitions and their operator and build call sites, then review the major-release constraint. Done means the project no longer supports `routine` and continues to support the `pod` strategy, with the affected behavior verified.

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