The sponge command has been imported on 2017-12-05 but the import has
been reverted the next day.

A script failed and I found that it was due to the left-over broken
sponge binary in base being prefered over the port version. To prevent
a known non-working binary to persist in /usr/bin, I'm adding sponge
to the obsolete files list even though it could only be installed on
a single day in 2017.

I do not plan to MFC this change since the issue will only exist on
systems installed from -CURRENT sources in 2017, and I do assume that
such systems are not running -STABLE today