Remove icons from explore nav

## What does this MR do?
Removes icons from explore `nav/_explore`

## Why was this MR needed?
To match signed-in nav

## Screenshots (if relevant)
Before:   
![Screen_Shot_2016-07-20_at_6.39.01_AM](/uploads/9ccc11d151e37a0ef2fab681ccc07efa/Screen_Shot_2016-07-20_at_6.39.01_AM.png)

After:   
![Screen_Shot_2016-07-20_at_6.43.15_AM](/uploads/ae058b19cc3c887bccf6c221ac58cfc2/Screen_Shot_2016-07-20_at_6.43.15_AM.png)


See merge request !5362