Remove is_coroutine as it is misleading and no longer needed

Author: ddfisher

mypy/checker.py

@@ -1497,11 +1497,6 @@ def visit_return_stmt(self, s: ReturnStmt) -> Type:
                         + ": expected {}, got {}".format(return_type, typ)
                     )
             else:
-                # return without a value
-                # empty returns are valid in coroutines
-                if (self.function_stack[-1].is_coroutine):
-                    return None
-
                 # empty returns are valid in Generators with Any typed returns
                 if (self.function_stack[-1].is_generator and isinstance(return_type, AnyType)):
                     return None

mypy/nodes.py

@@ -323,7 +323,6 @@ class FuncItem(FuncBase):
     # Is this an overload variant of function with more than one overload variant?
     is_overload = False
     is_generator = False   # Contains a yield statement?
-    is_coroutine = False   # Contains @coroutine or yield from Future
     is_static = False      # Uses @staticmethod?
     is_class = False       # Uses @classmethod?
     # Variants of function with type variables with values expanded

mypy/semanal.py

@@ -1447,7 +1447,6 @@ def visit_decorator(self, dec: Decorator) -> None:
                 self.check_decorated_function_is_method('abstractmethod', dec)
             elif refers_to_fullname(d, 'asyncio.tasks.coroutine'):
                 removed.append(i)
-                dec.func.is_coroutine = True
             elif refers_to_fullname(d, 'builtins.staticmethod'):
                 removed.append(i)
                 dec.func.is_static = True