[4/N][torch.compile] clean up set_torch_compile_backend (#10401)

Signed-off-by: youkaichao <youkaichao@gmail.com>

vllm/plugins/__init__.py

@@ -1,6 +1,6 @@
 import logging
 from contextlib import contextmanager
-from typing import TYPE_CHECKING, Callable, Optional, Union
+from typing import TYPE_CHECKING, Optional
 
 import vllm.envs as envs
 
@@ -50,18 +50,6 @@ def load_general_plugins():
                 logger.exception("Failed to load plugin %s", plugin.name)
 
 
-_torch_compile_backend: Optional[Union[Callable, str]] = None
-
-
-def set_torch_compile_backend(backend: Union[Callable, str]):
-    global _torch_compile_backend
-    _torch_compile_backend = backend
-
-
-def get_torch_compile_backend() -> Optional[Union[Callable, str]]:
-    return _torch_compile_backend
-
-
 _compilation_config: Optional[CompilationConfig] = None