Remove unused substitute.{h,cpp}

Single-character replacement should be done using std::replace. If full string
substitution is genuinely required, a more efficient implemention (not
repeatedly searching from the beginning) should be built.

Author: tautschnig

src/util/Makefile

@@ -80,7 +80,6 @@ SRC = arith_tools.cpp \
       string_container.cpp \
       string_hash.cpp \
       string_utils.cpp \
-      substitute.cpp \
       symbol.cpp \
       symbol_table_base.cpp \
       symbol_table.cpp \