tree-wide: Remove Emacs modelines

We added a `.dir-locals.el` in commit: 9a77017d87
There's no need to have it per-file, with that people might think
to add other editors, which is the wrong direction.

Closes: #1206
Approved by: jlebon

src/libostree/ostree-sysroot.h

@@ -1,5 +1,4 @@
-/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
- *
+/*
  * Copyright (C) 2013 Colin Walters <walters@verbum.org>
  *
  * This library is free software; you can redistribute it and/or