Unmuting fixed int4 flakiness (elastic#122545) (elastic#122602)

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

elastic#121437

closes: elastic#121412
(cherry picked from commit 4bba2ca)

Author: benwtrent

muted-tests.yml

@@ -450,9 +450,6 @@ tests:
 - class: org.elasticsearch.xpack.security.profile.ProfileIntegTests
   method: testSuggestProfileWithData
   issue: https://github.com/elastic/elasticsearch/issues/121258
-- 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.xpack.test.rest.XPackRestIT
   method: test {p0=transform/*}
   issue: https://github.com/elastic/elasticsearch/issues/120816