Removing sniffing in IFrameShim, more info in the PR

Since `MooTools 1.5` will have the Browser.* deprecated the `browsers` options has little to no meaning, 
so by now I defaulted it to `false`,
for the compat build it will stay `(Browser.ie6 || (Browser.firefox && Browser.version < 3 && Browser.Platform.mac))`
which are dead browsers by now,  I also wanted to rename it to something better like enable but I'm not even sure that Iframeshim is required nowadays anywhere