support/docker: drop Python 2.x modules

Since commit 4a40d36f
("support/testing: switch to Python 3 only") our runtime testing
infrastructure is Python 3.x only.

Therefore, it is no longer needed to have python-nose2 and
python-pexpect in the Docker container used to run our Gitlab CI jobs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>