Remove useless error FindEntittyError

This error is not used since the deprecated find_one_entity method has been removed in 566f8fce5168

cubicweb/req.py

@@ -34,10 +34,6 @@ ONESECOND = timedelta(0, 1, 0)
 CACHE_REGISTRY = {}
 
 
-class FindEntityError(Exception):
-    """raised when find_one_entity() can not return one and only one entity"""
-
-
 class Cache(dict):
     def __init__(self):
         super(Cache, self).__init__()

doc/api/req.rst

@@ -5,7 +5,5 @@
 
 .. automodule:: cubicweb.req
 
-    .. autoexception:: FindEntityError
-
     .. autoclass:: RequestSessionBase
         :members: