Remove reference to Makefile in README.

Author: jeschkies

postgres-mixin/README.md

@@ -15,12 +15,5 @@ $ go get github.com/monitoring-mixins/mixtool/cmd/mixtool
 $ go get github.com/google/go-jsonnet/cmd/jsonnetfmt
 ```
 
-You can then build the Prometheus rules files `alerts.yaml` and
-`rules.yaml` and a directory `dashboard_out` with the JSON dashboard files
-for Grafana:
-```bash
-$ make build
-```
-
 For more advanced uses of mixins, see
 https://github.com/monitoring-mixins/docs.