Fallback to default promu's crossbuild platforms list

.promu.yml

@@ -7,7 +7,7 @@ build:
           path: ./cmd/prometheus
         - name: promtool
           path: ./cmd/promtool
-    flags: -tags netgo
+    flags: -a -tags netgo
     ldflags: |
         -X {{repoPath}}/version.Version={{.Version}}
         -X {{repoPath}}/version.Revision={{.Revision}}
@@ -21,25 +21,3 @@ tarball:
         - documentation/examples/prometheus.yml
         - LICENSE
         - NOTICE
-crossbuild:
-    platforms:
-        - linux/amd64
-        - linux/386
-        - darwin/amd64
-        - darwin/386
-        - windows/amd64
-        - windows/386
-        - freebsd/amd64
-        - freebsd/386
-        - openbsd/amd64
-        - openbsd/386
-        - netbsd/amd64
-        - netbsd/386
-        - dragonfly/amd64
-        - linux/arm
-        - linux/arm64
-        - freebsd/arm
-        - openbsd/arm
-        - netbsd/arm
-        - linux/ppc64
-        - linux/ppc64le