Remove Dockerfiles

These Dockerfiles are no longer required due to the projects
being built upstream and published to Quay/Docker Hub.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

prometheus/Dockerfile.alertmanager

-FROM prom/alertmanager:v0.7.1
-
-COPY alertmanager.yml /alertmanager.yml

prometheus/Dockerfile.prometheus

-FROM prom/prometheus:v1.5.2
-
-COPY prometheus.yml /etc/prometheus/prometheus.yml
-COPY alert.rules /etc/prometheus/alert.rules
-