Remove PyYaml apt package

NBI is not building with PyYAML from apt, need to prefer the version from
pip instad.

Change-Id: Ie2bbf3a9c1dde60b8876f3bd2063dc1649f4904b
Signed-off-by: beierlm <mark.beierl@canonical.com>

docker/NBI/Dockerfile

@@ -22,7 +22,7 @@ FROM ubuntu:18.04
 
 RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get -y install curl software-properties-common
 
-RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y git python3 python3-dev python3-pip python3-yaml
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y git python3 python3-dev python3-pip
 
 RUN python3 -m pip install --upgrade pip