[Reland] Remove torchao.quantization.prototype

**Summary:** This has been deprecated for 3 release already (since
0.7.0). This commit actually breaks the BC.

**Test Plan:**
python test/quantization/test_qat.py

Author: andrewor14

test/quantization/test_qat.py

@@ -1133,62 +1133,6 @@ def embedding_forward_4w(x: torch.Tensor, weight: torch.Tensor) -> torch.Tensor:
         baseline_out = embedding_forward_4w(x2, fq_embedding.weight)
         torch.testing.assert_close(baseline_out, fq_out, atol=0, rtol=0)
 
-    @unittest.skipIf(
-        not TORCH_VERSION_AT_LEAST_2_4, "skipping when torch version is 2.4 or lower"
-    )
-    def test_qat_prototype_bc(self):
-        """
-        Just to make sure we can import all the old prototype paths.
-        We will remove this test in the near future when we actually break BC.
-        """
-        from torchao.quantization.prototype.qat import (  # noqa: F401, F811, I001
-            disable_4w_fake_quant,
-            disable_8da4w_fake_quant,
-            enable_4w_fake_quant,
-            enable_8da4w_fake_quant,
-            ComposableQATQuantizer,
-            Int8DynActInt4WeightQATLinear,
-            Int4WeightOnlyEmbeddingQATQuantizer,
-            Int4WeightOnlyQATQuantizer,
-            Int8DynActInt4WeightQATQuantizer,
-        )
-        from torchao.quantization.prototype.qat._module_swap_api import (  # noqa: F401, F811
-            disable_4w_fake_quant_module_swap,
-            enable_4w_fake_quant_module_swap,
-            disable_8da4w_fake_quant_module_swap,
-            enable_8da4w_fake_quant_module_swap,
-            Int4WeightOnlyQATQuantizerModuleSwap,
-            Int8DynActInt4WeightQATQuantizerModuleSwap,
-        )
-        from torchao.quantization.prototype.qat.affine_fake_quantized_tensor import (  # noqa: F401, F811
-            AffineFakeQuantizedTensor,
-            to_affine_fake_quantized,
-        )
-        from torchao.quantization.prototype.qat.api import (  # noqa: F401, F811
-            ComposableQATQuantizer,
-            FakeQuantizeConfig,
-        )
-        from torchao.quantization.prototype.qat.embedding import (  # noqa: F401, F811
-            FakeQuantizedEmbedding,
-            Int4WeightOnlyEmbeddingQATQuantizer,
-            Int4WeightOnlyEmbedding,
-            Int4WeightOnlyQATEmbedding,
-        )
-        from torchao.quantization.prototype.qat.fake_quantizer import (  # noqa: F401, F811
-            FakeQuantizer,
-        )
-        from torchao.quantization.prototype.qat.linear import (  # noqa: F401, F811
-            disable_4w_fake_quant,
-            disable_8da4w_fake_quant,
-            enable_4w_fake_quant,
-            enable_8da4w_fake_quant,
-            FakeQuantizedLinear,
-            Int4WeightOnlyQATLinear,
-            Int4WeightOnlyQATQuantizer,
-            Int8DynActInt4WeightQATLinear,
-            Int8DynActInt4WeightQATQuantizer,
-        )
-
     @unittest.skipIf(
         not TORCH_VERSION_AT_LEAST_2_4, "skipping when torch version is 2.4 or lower"
     )