Remove workflow-aggregator-plugin

Commit 9f607fc3d22e591ce7d03718277197fcea80b3fe introduced this plugin
instead of build-flow-plugin but there is no more usage of build-flow-plugin
since the gerrit trigger has been replaced by a pipeline job.
(Note that if there were still usages of it, commit
9f607fc3d22e591ce7d03718277197fcea80b3fe would have been breaking
them).

Change-Id: I86c5ae4e9cfc4305e9d773e6943baf38cbc2e2ab
diff --git a/jenkins/base/plugins.bzl b/jenkins/base/plugins.bzl
index 3be9d02..97f9a1e 100644
--- a/jenkins/base/plugins.bzl
+++ b/jenkins/base/plugins.bzl
@@ -138,14 +138,6 @@
         "1.36.2",
         "ecf7cd881136e60e48e220172bd0ddedb77c50129bc9dac271b32c715be38541",
     ],
-    "workflow-aggregator": [
-        "2.5",
-        "f5122377769a5e43ee4c2d88c720dd893054c3658fc4d29f21d362eabcaac86a",
-    ],
-    "pipeline-milestone-step": [
-        "1.3",
-        "15705a8b6e396ea8fd2dab935d7639bed429b2ac3fb6981ac6e02da63f0cb53b",
-    ],
     "embeddable-build-status": [
         "1.9",
         "9d950ce4bfdcb67e1b1198ea2b54e18de95ec3214b1cf1135141b5b54a62edb2",