Fix tests broken because future not completed during cleanup (#121782)

A future.actionGet was missing from the delete pipeline action execution in the test cleanup, causing all tests to fail intermittently. Also replace actionGet with safeGet.

Author: parkertimmins

muted-tests.yml

@@ -396,8 +396,6 @@ tests:
   issue: https://github.com/elastic/elasticsearch/issues/121465
 - class: org.elasticsearch.xpack.esql.qa.multi_node.EsqlSpecIT
   issue: https://github.com/elastic/elasticsearch/issues/121411
-- class: org.elasticsearch.xpack.migrate.action.ReindexDatastreamIndexTransportActionIT
-  issue: https://github.com/elastic/elasticsearch/issues/121737
 
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