Strip non-BMP characters from string in Mathematica <12.

stripNonBMPCharacters.m

surrogateQ[ch_] := # >= 55296 && # < 57344 &@First@ToCharacterCode[ch];
surrogateQ::usage = 
  "Tests whether the given character is a surrogate, i.e., in the \
range U+D800 to U+DFFF.";

stripNonBMPCharacters[s_] := 
  StringJoin[Select[Characters[s], ! surrogateQ[#] &]];
stripNonBMPCharacters::usage = 
  "Strips the given string of Unicode code points outside of the \
Basic Multilingual Plane (BMP), i.e., characters beyond U+FFFF, by \
removing UTF-16 surrogate pairs.";