Remove unused and unimplemented asyncio.Task class

This class originated from commit 142d08c and it is unused
since _PortageEventLoop was removed in commit 20204fd.

Fixes: 20204fd ("Remove unused _PortageEventLoop and _PortageChildWatcher")
Signed-off-by: Zac Medico <zmedico@gentoo.org>

lib/portage/util/futures/_asyncio/__init__.py

@@ -19,7 +19,6 @@
     "run",
     "shield",
     "sleep",
-    "Task",
     "wait",
     "wait_for",
 )
@@ -174,16 +173,6 @@ def __init__(self, **kwargs):
         super().__init__(**kwargs)
 
 
-class Task(Future):
-    """
-    Schedule the execution of a coroutine: wrap it in a future. A task
-    is a subclass of Future.
-    """
-
-    def __init__(self, coro, loop=None):
-        raise NotImplementedError
-
-
 def ensure_future(coro_or_future, loop=None):
     """
     Wrap a coroutine or an awaitable in a future.