Quick fix for naked url imports

[seven-phases-max]

Fixes #2516.

Actually I'm not quite sure: maybe this simplification is too liberal but I did not find any weird consequences except that things like @import ~"some.less"; now work too. Any other value types won't pass through the parser anyway.

[lukeapage]

Did you check the history to see if its a regression?

[seven-phases-max]

No, I did not. But I have right now: it looks like this was changed in 1.4.0. In 1.3.3 it was like this - though I'm not sure if it's the same piece of code actually... No, my mistake, it's actually a regression of v2 - in 1.7.5 it works as expected. I did not find corresponding commit though.

P.S. It looks like the code in this function was the same for a while, but the breaking change was made elsewhere in the code. Unfortunately I can't find what exactly could affect this.

[lukeapage]

Anyway, looks fine..