Tests: Remove is_true check in REST test

If the duration time was 0 (and this might happen due to
using System.currentTimeMillis), the is_true check
still returns false.

The correct fix will be done later to replace the offending
measurement calls and replace them. Then we can add back this
line.

Original commit: elastic/x-pack-elasticsearch@076a9a37cc

plugin/src/test/resources/rest-api-spec/test/watcher/execute_watch/10_basic.yml

@@ -61,7 +61,6 @@ teardown:
   - match: { watch_record.status.state.active: true }
   - is_true: watch_record.node
   - match: { watch_record.status.actions.indexme.ack.state: "ackable" }
-  - is_true: watch_record.result.execution_duration
 
 ---
 "Test execute watch API with user supplied watch":