git-cola-folder-handler.desktop: fix desktop-file-validate warnings

Use %f instead of %u since we expect local paths in the file handler.

Remove the "Path" entry since it is not really needed and is using
%u, which is not a valid value.

Closes #356

Reported-by: Justin Lecher <jlec@gentoo.org>
Signed-off-by: David Aguilar <davvid@gmail.com>