remove UPLOAD_DATA_DIR from configuration example.

[sjanssen2]

trafficstars

I might miss something here, but from reading the code of https://github.com/qiita-spots/qiita/blob/fdad618cc6283430decc6511c0cc6b9b477c1fb6/qiita_core/configuration_manager.py#L21 I cannot spot where the field UPLOAD_DATA_DIR of the [main] section is ever used. Maybe you want to remove it?

https://github.com/qiita-spots/qiita/blob/fdad618cc6283430decc6511c0cc6b9b477c1fb6/qiita_core/support_files/config_test.cfg#L28-L30

also in the test

https://github.com/qiita-spots/qiita/blob/fdad618cc6283430decc6511c0cc6b9b477c1fb6/qiita_core/tests/test_configuration_manager.py#L309-L311