Remove redundant last push event if activity feed is set to main page

### What does this MR do?

This MR eliminates a redundant last push event message if the user's preferences are set to the main activity feed.

### Why was this MR needed?

Lots of wasted space was introduced as a result.

### Screenshots

#### Before

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/e7fdeb58b2017ec45443bf0d54a94685/image.png)

#### After

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/d4a15113127be119e7c33ff501020f16/image.png)

### What are the relevant issue numbers?

Closes #2009

See merge request !995