Remove NioNotEnabledBootstrapCheck bootstrap check (#28901)

This is related to #27260. This commit removes the bootstrap check that
prevents nio from being enabled.

Author: Tim-Brooks

plugins/transport-nio/src/main/java/org/elasticsearch/transport/nio/NioNotEnabledBootstrapCheck.java

@@ -61,9 +61,4 @@ public Map<String, Supplier<Transport>> getTransports(Settings settings, ThreadP
             () -> new NioTransport(settings, threadPool, networkService, bigArrays, pageCacheRecycler, namedWriteableRegistry,
                 circuitBreakerService));
     }
-
-    @Override
-    public List<BootstrapCheck> getBootstrapChecks() {
-        return Collections.singletonList(new NioNotEnabledBootstrapCheck());
-    }
 }