Jenkinsfile: don't use Map.remove() as it is forbidden

When retriggering flaky builds, do not use the non-CPS method Map.remove()
and instead create a new Map.

Bug: Issue 11876
Change-Id: I383a2351ad907acd19d21c459a0fd660e8abe028

Jenkinsfile

@@ -196,7 +196,7 @@ def findFlakyBuilds() {
     def retryBuilds = []
     flaky.each {
         def mode = it.key
-        Builds.verification.remove(mode)
+        Builds.verification = Builds.verification.findAll { it.key != mode }
         retryBuilds += mode
     }