Remove duplicate/conflicting command from tutorial

[rolfedh]

The first command in the tutorial, https://revel.github.io/tutorial/createapp.html, sets the GOPATH to a different value than the value given in https://revel.github.io/tutorial/gettingstarted.html#set_up_your_gopath.

This might confuse users and seems like an unnecessary duplication.

Would it make sense to remove the following line from gettingstarted.md?

$ export GOPATH="/home/me/gostuff"

[rolfedh]

If we need to remind users to set their GOPATH correctly, we could add a prerequisite like this:
Prerequisite: [Configure your GOPATH](https://revel.github.io/tutorial/gettingstarted.html#set_up_your_gopath).