Revert "[Add] metaclass of singleton"

This reverts commit b05eb7142e.

vnpy/trader/utility.py

@@ -445,17 +445,3 @@ def virtual(func: "callable"):
     that can be (re)implemented by subclasses.
     """
     return func
-
-
-class Singleton(type):
-    """
-    Metaclass for creating singleton object.
-    """
-    _instances = {}
-
-    def __call__(cls, *args, **kwargs):
-        """"""
-        if cls not in cls._instances:
-            cls._instances[cls] = super(
-                Singleton, cls).__call__(*args, **kwargs)
-        return cls._instances[cls]