Remove confusing empty diff message.

The existing message made assumptions (e.g. that `==` was used for comparison)
there were not always true, causing confusion.

Fixes #143.

Note that this reverts the fix for #123.  I think that in the context of the differ, it does not have enough context (e.g. about what matcher was used) to give a message that makes sense in all situations.  @alindeman / @dchelimsky -- are you guys OK with this?  Or is there a better way to solve this issue?