Remove test/levelup-test.js

[ralphtheninja]

Closes #96

[ralphtheninja]

Also see Level/level-browserify#47

[vweevers]

We can also rewrite the destroy tests to not use levelup. This:

level-js/test/custom-test.js

Line 168 in 2666a50

test('test levelup .destroy', function (t) {

And

level-js/test/custom-test.js

Line 191 in 2666a50

test('test levelup .destroy and custom prefix', function (t) {

Then we can remove the levelup dependency as well as the pinkie Promise polyfill.

[ralphtheninja]

@vweevers Yes, let's do that.