Stop disabling disgnostic trust manager in settings (#56916)

Now that #56526 is merged, we do not need to explicitly disable
the diagnostic trust manager for all of our test clusters - we do
this dynamically in runtime if the combination of java version and
JSSE provider dictates that.

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -161,8 +161,6 @@ class BuildPlugin implements Plugin<Project> {
                         cluster.systemProperty('javax.net.ssl.trustStorePassword', 'password')
                         cluster.systemProperty('javax.net.ssl.keyStorePassword', 'password')
                         cluster.systemProperty('javax.net.ssl.keyStoreType', 'BCFKS')
-                        // Can't use our DiagnosticTrustManager with SunJSSE in FIPS mode
-                        cluster.setting 'xpack.security.ssl.diagnose.trust', 'false'
                     }
                 }
             }