simplify backend implementation of dpnp.kaiser

[vtavana]

This PR simplifies backend implementation of dpnp.kaiser by getting rid of unnecessary template and address some technical debt.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/index.html

[coveralls]

coverage: 71.705%. remained the same
when pulling b5c8515 on simplify-kaiser
into b10bd15 on master.

[github-actions]

Array API standard conformance tests for dpnp=0.19.0dev0=py312h509198e_18 ran successfully.
Passed: 1231
Failed: 0
Skipped: 9

[antonwolfy]

Thank you @vtavana

[antonwolfy]

The issues in internal CI seem unrelating, so merging.