gnu: go-github-com-prometheus-client-model: Simplify.

* gnu/packages/prometheus.scm (go-github-com-prometheus-client-model)
[arguments] <skip-build?>: No go files in project's root.
<tests?>: Enable them.
<phases>: Do not delete 'build, skip it.

Change-Id: Iee59390189b237977ca3b4b163721ee11caa07aa

gnu/packages/prometheus.scm

@@ -238,12 +238,8 @@ Prometheus metrics.")
     (build-system go-build-system)
     (arguments
      (list
-      #:import-path "github.com/prometheus/client_model"
-      #:tests? #f
-      #:phases
-      #~(modify-phases %standard-phases
-        ;; Source-only package
-        (delete 'build))))
+      #:skip-build? #t
+      #:import-path "github.com/prometheus/client_model"))
     (propagated-inputs
      (list go-github-com-golang-protobuf))
     (home-page "https://github.com/prometheus/client_model")