remove unused exception.

Author: Vizonex

src/anyio/_core/_exceptions.py

@@ -184,13 +184,6 @@ class FutureCancelled(Exception):
     """
 
 
-class FutureNotFinished(Exception):
-    """
-    Raised when attempting to access the return value or exception of a
-    :class:`.Future` that is still pending completion.
-    """
-
-
 class FutureAlreadyFinished(Exception):
     """
     Raised when attempting set a result of or await a