util/StringUtil: move CopyString() to TruncateString.cxx

src/util/StringUtil.hxx

@@ -24,18 +24,6 @@
 
 #include <stddef.h>
 
-/**
- * Copy a string.  If the buffer is too small, then the string is
- * truncated.  This is a safer version of strncpy().
- *
- * @param size the size of the destination buffer (including the null
- * terminator)
- * @return a pointer to the null terminator
- */
-gcc_nonnull_all
-char *
-CopyString(char *dest, const char *src, size_t size) noexcept;
-
 /**
  * Returns a pointer to the first non-whitespace character in the
  * string, or to the end of the string.