Unmuting fixed int4 flakiness (#122545)

This test was missed back in my "fix int4" PR. 

#121437

closes: #121412

Author: benwtrent

muted-tests.yml

@@ -326,9 +326,6 @@ tests:
 - class: org.elasticsearch.ingest.geoip.FullClusterRestartIT
   method: testGeoIpSystemFeaturesMigration {cluster=UPGRADED}
   issue: https://github.com/elastic/elasticsearch/issues/121115
-- class: org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT
-  method: test {p0=search.vectors/42_knn_search_int4_flat/Vector similarity with filter only}
-  issue: https://github.com/elastic/elasticsearch/issues/121412
 - class: org.elasticsearch.smoketest.DocsClientYamlTestSuiteIT
   method: test {yaml=reference/cat/health/cat-health-no-timestamp-example}
   issue: https://github.com/elastic/elasticsearch/issues/121867