[Refactor][Config] Remove transpose KV cache environment variable (#14832)

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

Removes the legacy `VLLM_ASCEND_FUSION_OP_TRANSPOSE_KV_CACHE_BY_BLOCK`
environment variable after its behavior was migrated to
`additional_config.enable_transpose_kv_cache_by_block`. 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_FUSION_OP_TRANSPOSE_KV_CACHE_BY_BLOCK` is no longer
read. Users must configure the behavior with `--additional-config
'{"enable_transpose_kv_cache_by_block":true}'`.

### 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_FUSION_OP_TRANSPOSE_KV_CACHE_BY_BLOCK` remain outside
migration documentation and the ignored-variable unit test
- 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>