[Refactor][Config] Remove weight NZ environment variable (#14830)

### What this PR does / why we need it?

Removes the legacy `VLLM_ASCEND_ENABLE_NZ` environment variable after
its behavior was migrated to `additional_config.weight_nz_mode`. Runtime
code, deployment examples, test configurations, and documentation are
migrated to the config field where applicable.

This is one independently reviewable part of the seven-variable split of
#14637.

### Does this PR introduce _any_ user-facing change?

Yes. `VLLM_ASCEND_ENABLE_NZ` is no longer read. Users must configure the
behavior with `--additional-config '{"weight_nz_mode":1}'`.

### How was this patch tested?

- `ruff check` and `ruff format --check` on changed Python files
- Python byte-compilation of changed Python files
- YAML parsing of changed YAML files
- JSON parsing of concrete `additional-config` payloads
- Scan confirming no active references to `VLLM_ASCEND_ENABLE_NZ` remain
outside migration documentation
- NPU end-to-end tests were not run in this Windows environment

- vLLM version: v0.27.1
- vLLM main:
https://github.com/vllm-project/vllm/commit/ba07e4a48fc951300d97eb506217dd530583dea3

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Signed-off-by: kk_1999 <97623903+kk-ss1999@users.noreply.github.com>