remove feature check for Presto, simplified code fixes test on IE6-8

removing this check [as discussed](https://github.com/mootools/mootools-core/commit/912a23600d64d9c8030a58c700ac6617e09475ca#commitcomment-5228993), this code was intended to avoid using `getComputedStyle` on Presto as that engine used to round sub-pixel sizes. Now that Presto is gone in favor of Blink this check doesn't make any more sense (this code didn't get as part of a release yet, so we shouldn't even expect regressions other than a broken test on Presto)