git-gui: Remove 'Allow Partially Added Files' option.

git-gui: Remove 'Allow Partially Added Files' option.

Now that we take the approach of core Git where we allow the user to
stage their changes directly into the index all of the time there is
absolutely no reason to have the Allow Partially Added Files option,
nor is there a reason or desire to default that option to false.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>