tests: remove unused _TestIntegerValue.test_create_neg

tests: remove unused _TestIntegerValue.test_create_neg

This test method is unused, since all sub-classes define a
test_create_neg method.  I confirmed this by:

 - placing an `assert False` inside it and running the test
 - comparing the number of tests ran as reported by the unittest runner,
   before and after the change

Change-Id: Ib2bd1cb919c85e6dddc21c0f305cfab8504d06e9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11409
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>