avocado/core/output.py: remove unused PagerNotFoundError exception

It was introduced in 8f8d668b, but stoped being used in 6681f2d6.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/output.py

@@ -518,10 +518,6 @@ class MemStreamHandler(logging.StreamHandler):
         """
 
 
-class PagerNotFoundError(Exception):
-    pass
-
-
 class Paginator(object):
 
     """