[hardware] unify usage of is_tpu to current_platform.is_tpu() (#7102)

vllm/utils.py

@@ -333,15 +333,6 @@ def is_neuron() -> bool:
     return transformers_neuronx is not None
 
 
-@lru_cache(maxsize=None)
-def is_tpu() -> bool:
-    try:
-        import libtpu
-    except ImportError:
-        libtpu = None
-    return libtpu is not None
-
-
 @lru_cache(maxsize=None)
 def is_xpu() -> bool:
     from importlib.metadata import PackageNotFoundError, version