[BUG] DBSCAN: Fix or remove algorithm flags

[ahendriksen]

Describe the bug
The DBSCAN algorithm in its current form has some flags that determine the algorithm that is used in the calculation of the vertex degree and adjacency matrix. Currently, both steps have only one algorithm.

The code should be cut down quite a bit more:

1. The vertex degree step has two mostly similar algorithms for precomputed and other metrics. This could (should?) be unified;
2. The adj_graph step now has a single algorithm and a separate runner file that is redundant.

Additional context
PR #4804 removes the naive versions of the algorithms. Code review: #4804 (comment)

[github-actions]

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