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C#6, 201 bytes, by milk

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

trueJoin is not defined, so only string.Concat() 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.