Drop MANIFEST.in - it's not needed by pbr

diskimage-builder already uses PBR:-
setuptools.setup(
    setup_requires=['pbr>=1.8'],
    pbr=True)

This patch removes `MANIFEST.in` file as pbr generates a
sensible manifest from git files and some standard files
and it removes the need for an explicit `MANIFEST.in` file.

Change-Id: Iad8b724c1d1e0ef358606b13da4569d881ec740a

MANIFEST.in

@@ -1,6 +0,0 @@
-include bin/disk-image-create
-include bin/element-info
-include bin/ramdisk-image-create
-graft lib
-graft elements
-include README.rst