Remove an extra assertion

As the result of https://bugs.ruby-lang.org/issues/13043, now
Exception#cause should not have a loop.

In the example jimweirich#272, the
code doesn't seem to intend the loop itself but just re-raising
the first exception instead of the next exception.  Therefore I
consider the last assertion superfluous.

Author: nobu

test/test_rake_application.rb

@@ -97,7 +97,6 @@ def test_display_exception_details_cause_loop
     assert_empty out
 
     assert_match "cause a", err
-    assert_match "cause b", err
   end
 
   def test_display_tasks