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

octavia 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: I0b6a7a6eb3be8c1982a4fd7576dea1db9890f69c
Closes-Bug: #1608980

MANIFEST.in

@@ -1,6 +0,0 @@
-include README.rst
-include LICENSE
-
-exclude .gitignore
-exclude .gitreview
-global-exclude *.pyc