[CUDA] Deprecate SkipLayerNorm strict mode (#29388)
### Description
Removes the deprecated `enable_skip_layer_norm_strict_mode` CUDA
provider option and its associated "strict mode" code path from the CUDA
`SkipLayerNorm` kernel.
Strict mode previously routed `SkipLayerNorm` through the
`LayerNormalization` kernel to gain fp32-accumulation accuracy at the
cost of performance. After #28682 made `SkipLayerNorm`/`EmbedLayerNorm`
CUDA kernels always accumulate in fp32, the strict-mode path is
redundant: the default kernel already provides the same accuracy with
better performance. This change deletes the now-dead branch and the
plumbing that fed it.
### Key Changes
- **`skip_layer_norm.cc`**: Remove the `strict_` branch that called
`HostApplyLayerNorm`; always launch `LaunchSkipLayerNormKernel`. Drop
the now-unused `layer_norm_impl.h` include and the input/skip same-shape
strict-mode shape check.
- **`skip_layer_norm.h`**: Remove the `strict_` member.
- **`cuda_execution_provider.h`**: Remove
`IsSkipLayerNormInStrictMode()`.
- **`cuda_kernel_adapter.h`**: Remove
`GetCudaKernelAdapterSkipLayerNormStrictMode()` shim.
- **`cuda_provider_options.h`**: Keep
`enable_skip_layer_norm_strict_mode` field for ABI/back-compat but mark
it deprecated and ignored.
- **`skiplayernorm_op_test.cc`**: Drop the redundant strict-mode test
passes; tests now run a single default path.
The provider option is retained (ignored) to preserve backward
compatibility — existing configs that set it continue to work without
error.
### Motivation
Follow-up cleanup to #28682, which switched the CUDA kernels to fp32
accumulation, making strict mode obsolete.
### Testing
- `skiplayernorm_op_test.cc` covers fp16/fp32/bf16 default path;
strict-mode passes removed.
