Merge branch 'users-should-remove-former-go-installation-folder' into 'master'

users should remove former Go installation folder

## What does this MR do?

It ensure users installing or updating from source will remove their former Go installation folder in `/usr/local/go`, to avoid raising errors that could lead to new issues being created.

## Are there points in the code the reviewer needs to double check?

1. The `rm -rf` should not be verbose if folder does not exists.
1. May a user have some kind of configuration files in this folder ?

## Why was this MR needed?

I did faced the error, that led me to the solution stated here : https://gitlab.com/gitlab-org/gitlab-workhorse/issues/29#note_4155517

gitlab-org/gitlab-workhorse#29

See merge request !6602

doc/install/installation.md

@@ -142,6 +142,9 @@ gitlab-workhorse we need a Go compiler. The instructions below assume you
 use 64-bit Linux. You can find downloads for other platforms at the [Go download
 page](https://golang.org/dl).
 
+    # Remove former Go installation folder
+    sudo rm -rf /usr/local/go
+
     curl --remote-name --progress https://storage.googleapis.com/golang/go1.5.3.linux-amd64.tar.gz
     echo '43afe0c5017e502630b1aea4d44b8a7f059bf60d7f29dfd58db454d4e4e0ae53  go1.5.3.linux-amd64.tar.gz' | shasum -a256 -c - && \
       sudo tar -C /usr/local -xzf go1.5.3.linux-amd64.tar.gz