Remove unnecessary left pane modules

Created by: benvenker

There are lots of home panels. Some of them are intended to be educational but it’s not clear from data + qualitative feedback about Sourcegraph usability that they actually help users learn.

The remaining panels should be:

• Repositories
• Dynamic filters

Note: The goal of the Lucky search work is to make it so users don’t have to learn the query language (but can always drop into it if they need to), which would greatly reduce or eliminate the need for this type of product education being so front and center.

Another note: We believe the correct answer here is to use one solution for quickly getting to a scoped search: contexts. We will make future efforts to improve the UX but even now it's easier to do than the other methods, and ultimately just as accessible in the product.