chore: remove unused nrn variable from base module

[agustincelentano]

Summary

• Removes variable "nrn" from nullplatform/base/variables.tf (was declared but never consumed by any resource).
• Removes the nrn_without_namespace local from nullplatform/base/locals.tf (its only use was its own definition; no downstream reference).

Why

The base module accepted nrn as input but none of its resources (two kubernetes_namespace_v1 and a helm_release) used the value. The template rendered into the helm release does not include NRN either. Passing the variable created a false signal that base needed to be re-applied when the account changed — it didn't.

Breaking change

Consumers still passing nrn = ... to module "base" will get:

An argument named "nrn" is not expected here.

They should drop that argument. Same pattern as the np_api_key cleanups already shipped in v1.53.0 for other modules.

Test plan

• Verify `tofu plan` on a consumer that currently passes `nrn` shows the `unexpected argument` error.
• Verify `tofu plan` after dropping `nrn` from the consumer shows no diff in `module.base` resources.