Remove unused using (#501)

Remove unused using directive flagged by clang-tidy.

Author: roccomoretti

source/src/apps/public/motif_ligand_discovery/remove_duplicate_motifs.cc

@@ -69,7 +69,6 @@
 
 using namespace protocols::dna;
 using utility::vector1;
-using utility::string_split;
 
 using namespace core;
 using namespace basic;