Remove some redundant i18n

This is my first time using Rails i18n, so I am bound to make some
mistakes. Apparently, the Rails FormBuilder will automatically look up
the model attribute in the i18n configuration for the label.
Additionally, i18n strings have some sensible defaults. Knowing these 2
bits of information I was able to remove some redundant code in the
sweeps form and the en.yml configuration file.

Change-Id: I1fea91f7247e62a23721b1573eee20659dad990b