Remove _as_pauli_string

Since it is unused now, and __neg__ could use _try_interpret_as_pauli_string
instead.

Author: ToastCheng

cirq-core/cirq/ops/pauli_string.py

@@ -1134,11 +1134,8 @@ def qubit(self) -> raw_types.Qid:
         assert len(self.qubits) == 1
         return self.qubits[0]
 
-    def _as_pauli_string(self) -> PauliString:
-        return PauliString(qubit_pauli_map={self.qubit: self.pauli})
-
     def __neg__(self):
-        return -self._as_pauli_string()
+        return -_try_interpret_as_pauli_string(self)
 
     def _json_dict_(self) -> dict[str, Any]:
         return protocols.obj_to_dict_helper(self, ['pauli', 'qubit'])