Remove warning message when show import page

Fixes: https://github.com/MetaMask/metamask-extension/issues/1192

ui/app/accounts/import/index.js

@@ -72,6 +72,7 @@ AccountImportSubview.prototype.render = function () {
             }
           }),
           onChange: (opt) => {
+            props.dispatch(actions.showImportPage())
             this.setState({ type: opt.value })
           },
         }),

ui/app/reducers/app.js

@@ -122,6 +122,7 @@ function reduceApp (state, action) {
           name: 'import-menu',
         },
         transForward: true,
+        warning: null,
       })
 
     case actions.SHOW_INFO_PAGE: