bug: the upstream used in the plugin traffic-split can be deleted

[highfly22]

Issue description

When deleting a upstream, the admin service doesn't check whether it is used in the traffic-split plugin.

Environment

• apisix version docker image apache/apisix:2.10.0-alpine
• OS (cmd: uname -a): Linux algorithm-gateway-prod-apisix-57dcf5569f-v9gdj 3.10.0-1160.15.2.el7.x86_64 change: added doc of how to load plugin. #1 SMP Wed Feb 3 15:06:38 UTC 2021 x86_64 Linux

Steps to reproduce

1. create traffic-split route and assign a upstream to the route
2. Delete the upstream

Actual result

The upstream had been removed.

Error log

Code 200

Expected result

Then admin service would reject the delete operation.

[tokers]

Would you like to submit a PR to fix this bug? Thanks!