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` for the same thing.

(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. This is how [Underscore does it][u].)

[u]: https://github.com/mirven/underscore.lua/blob/master/lib/underscore.lua#L335