certs manager test: remove unused codes

The "delete_" function is unused. It doesn't follow the naming
convention as well, which prevents us enable clang-tidy. See the child
patch.

This commit just removed it.

Tested: unit test passed.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I1f587c7b5e1af0eeedc9aac882be285e322bc23b
diff --git a/test/certs_manager_test.cpp b/test/certs_manager_test.cpp
index f7d5463..da08996 100644
--- a/test/certs_manager_test.cpp
+++ b/test/certs_manager_test.cpp
@@ -224,10 +224,6 @@
     {
         manager->install(path);
     }
-    void delete_()
-    {
-        manager->deleteAll();
-    }
 
     std::string generateCSR(std::vector<std::string> alternativeNames,
                             std::string challengePassword, std::string city,