Remove /asset from exportPathMap (#6053)

For some reason one test inconsistently failed.

test/integration/export/next.config.js

@@ -7,7 +7,6 @@ module.exports = (phase) => {
       return {
         '/': { page: '/' },
         '/about': { page: '/about' },
-        '/asset': { page: '/asset' },
         '/button-link': { page: '/button-link' },
         '/get-initial-props-with-no-query': { page: '/get-initial-props-with-no-query' },
         '/counter': { page: '/counter' },