Removing git fetch --all (#2083)

## Changes

- Original change: ```git fetch --all``` fetches all remotes. The cheat
sheet stated that it's fetching all branches.
- Updated change: Remove the entry altogether since it's rarely used and
therefore might not be well placed in a cheat sheet.

## Context

A colleague of mine pointed out, that ```git fetch --all``` doesn't
fetch all branches but all remotes.
```git fetch --help``` confirms that.
After discussing in the PR I now suggest to remove it with an updated change.

Author: dscho

content/cheat-sheet/_index.html

@@ -735,10 +735,6 @@ <h3>Fetch changes and then merge them into your current branch:</h3>
       <span class="or">OR</span>
       <code>git pull</code>
     </div>
-    <div class="item">
-      <h3>Fetch all branches:</h3>
-      <code>git fetch --all</code>
-    </div>
   </section>
 
   <section>