[Rest Api Compatibility] Enable parent_join inner_hits test (#75560)

The test in 7.x was fixed in #75534

modules/parent-join/build.gradle

@@ -19,9 +19,3 @@ restResources {
     include '_common', 'bulk', 'cluster', 'nodes', 'indices', 'index', 'search'
   }
 }
-
-tasks.named("yamlRestCompatTest").configure {
-  systemProperty 'tests.rest.blacklist', [
-    '/30_inner_hits/Test two sub-queries with only one having inner_hits'
-  ].join(',')
-}