remove literal check

https://github.com/emotion-js/facepaint/blob/master/src/index.js#L22-L24

Here, literal is not required, as on [line 22](https://github.com/emotion-js/facepaint/compare/master...bhanuprasadcherukuvada:facepaint:literal_check_rm?expand=1#diff-bfe9874d239014961b1ae4e89875a6155667db834a410aaaa2ebe3cf89820556L22) if the item is not an Array then we convert it to Array.
So, checking if the item is an array or not should be sufficient.

#28