Remove unnecessary test from util.deepcompare

The immediately preceding test has made sure that `ty1` and `ty2` are the
same.

As a result, if `ty1 ~= 'table'`, there's no need to test `ty2`.

(I know that this is awfully minor, but in the process of comparing the `deepcompare` functions of Penlight, Underscore and luassert, I noticed this and thought it might be worth mentioning.)