Revert "feat(plugin-vue): define __VUE_SSR__ flag"

This reverts commit 49618c1.

Author: yyx990803

packages/plugin-vue/src/index.ts

@@ -119,7 +119,6 @@ export default function vuePlugin(rawOptions: Options = {}): Plugin {
         define: {
           __VUE_OPTIONS_API__: true,
           __VUE_PROD_DEVTOOLS__: false,
-          __VUE_SSR__: !!config.build?.ssr,
           ...config.define
         },
         ssr: {