Remove hard-code

Author: kotamatsuoka

pandas/core/internals/blocks.py

@@ -1006,8 +1006,7 @@ def coerce_to_target_dtype(self, other):
         and will receive the same block
         """
         if isinstance(other, str):
-            dtype = "string"
-            if is_dtype_equal(self.dtype, dtype):
+            if infer_dtype_from(self.dtype, pandas_dtype=True):
                 return self
             else:
                 dtype = np.object_