Fix checkstyle violation in remove plugin tests

This commit fixes a redundant modifier checkstyle violation in
RemovePluginCommandTests.java.

distribution/tools/plugin-cli/src/test/java/org/elasticsearch/plugins/RemovePluginCommandTests.java

@@ -51,7 +51,7 @@ public class RemovePluginCommandTests extends ESTestCase {
 
         final Environment env;
 
-        public MockRemovePluginCommand(final Environment env) {
+        private MockRemovePluginCommand(final Environment env) {
             this.env = env;
         }