feat: remove getTips() api

[laumair]

Description of change

IRI version 1.8.6 removes getTips() endpoint. This commit removes the implementation from this lib.

Related: iotaledger/iri#1851

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• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How the change has been tested

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Change checklist

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• My code follows the contribution guidelines for this project
• I have performed a self-review of my own code
• [] I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• [] I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes