## C#6, 201 bytes, [by milk](http://codegolf.stackexchange.com/a/103202/60948)

<!-- language-all: lang-cs -->

    using System.Linq;
    _=>string./**/
    #if trueJoin
    (/*"*//*/""/**//*/"*/",
    #else
    Concat(//*/,
    #endif
    from m in" "let p=$@"{@" 
    "[1]}"from i in(_.Replace(p,@""))where!new[]{p[0]^32^10}.Contains(i)select@i);

[repl.it demo](https://repl.it/EqtQ/0)

`trueJoin` is not defined, so only `string.Join()` remains.  
`p` is `"\n"`. `p[0]^32^10` is space. It would have been (a bit) more difficult if it's `p[0]^42` or other similar constants.