Refactoring: Remove redundant subtypes attribute of TypeInfo (#4079)

Author: JukkaL

mypy/fixup.py

@@ -54,9 +54,6 @@ def visit_type_info(self, info: TypeInfo) -> None:
                 info.defn.accept(self)
             if info.names:
                 self.visit_symbol_table(info.names)
-            if info.subtypes:
-                for st in info.subtypes:
-                    self.visit_type_info(st)
             if info.bases:
                 for base in info.bases:
                     base.accept(self.type_fixer)

mypy/nodes.py

@@ -1863,7 +1863,6 @@ class is generic then it will be a type constructor of higher kind.
     declared_metaclass = None  # type: Optional[mypy.types.Instance]
     metaclass_type = None  # type: Optional[mypy.types.Instance]
 
-    subtypes = None  # type: Set[TypeInfo] # Direct subclasses encountered so far
     names = None  # type: SymbolTable      # Names defined directly in this type
     is_abstract = False                    # Does the class have any abstract attributes?
     is_protocol = False                    # Is this a protocol class?
@@ -1970,7 +1969,6 @@ def __init__(self, names: 'SymbolTable', defn: ClassDef, module_name: str) -> No
         self.names = names
         self.defn = defn
         self.module_name = module_name
-        self.subtypes = set()
         self.type_vars = []
         self.bases = []
         # Leave self.mro uninitialized until we compute it for real,