Reduce files included in MANIFEST.in.

Remove the following files from the distribution, by removing
them from MANIFEST.in:
- `*.aiida` export files, used for testing
- `*.png` files from the `docs` directory
- files in `utils`, with the exception of `fasentrypoints.py`
  which is used in the install process.

MANIFEST.in

@@ -1,9 +1,6 @@
-include aiida/backends/tests/export_import_test_files/*.aiida
-include aiida/backends/sqlalchemy/migrations/script.py.mako
 include aiida/cmdline/templates/*.tpl
 include aiida/manage/backup/backup_info.json.tmpl
-include docs/source/images/*.png
-include utils/*
+include utils/fastentrypoints.py
 include setup.json
 include AUTHORS.txt
 include CHANGELOG.md