[Web Portal] Remove entry for outdated link

[abuccts]

Remove entry for preview submission and old marketplace.

Close #2356.

[coveralls]

Coverage remained the same at 53.345% when pulling 7b5abe2 on xiongyf/remvoe-entry into 1e1b6bc on master.

[coveralls]

Coverage remained the same at 53.345% when pulling 7b5abe2 on xiongyf/remvoe-entry into 1e1b6bc on master.

[mslichao]

Seems src\app\plugin\plugin.component.js used <pai-plugin> which is MarketplaceElement, and MarketplaceElement required template-view.component which required githubThrottled.
Should we fix this too?

[abuccts]

plugin.component.js is just the plugin instead of MarketplaceElement, it doesn't require marketplace.