zulip_bots: Use package_data instead of MANIFEST.in.

This commit removes generate_manifest.py and package data files
are now included using the package_data kwarg to setup().

This is because, in certain situations, MANIFEST.in is a bit
finicky. For instance installing a package using:

python setup.py install

doesn't include files specified in MANIFEST.in, while using

pip install ./zulip_bots

does. package_data doesn't pose this problem, ergo it's better
for us.

tools/provision

@@ -11,8 +11,6 @@ CURRENT_DIR = os.path.dirname(os.path.abspath(__file__))
 ZULIP_BOTS_DIR = os.path.join(CURRENT_DIR, '..', 'zulip_bots')
 sys.path.append(ZULIP_BOTS_DIR)
 
-import generate_manifest
-
 red = '\033[91m'
 green = '\033[92m'
 end_format = '\033[0m'