Docker: Python requirements are now managed in-tree

Neither mbed-host-tests nor mock are currently used.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/docker/bionic/Dockerfile b/tests/docker/bionic/Dockerfile
index 41789c6..3da14a2 100644
--- a/tests/docker/bionic/Dockerfile
+++ b/tests/docker/bionic/Dockerfile
@@ -161,6 +161,4 @@
 ENV GNUTLS_NEXT_CLI=/usr/local/gnutls-3.7.2/bin/gnutls-cli
 ENV GNUTLS_NEXT_SERV=/usr/local/gnutls-3.7.2/bin/gnutls-serv
 
-RUN pip3 install --no-cache-dir \
-    mbed-host-tests \
-    mock
+RUN scripts/min_requirements.py