Remove confusing sc_str_truncate()

This util function was error-prone:
 - it accepted a buffer as parameter (not necessarily a NUL-terminated
   string) and its length (including the NUL char, if any);
 - it wrote '\0' over the last character of the buffer, so the last
   character was lost if the buffer was not a NUL-terminated string, and
   even worse, it caused undefined behavior if the length was empty;
 - it returned the length of the resulting NUL-terminated string,
   which was inconsistent with the input buffer length.

In addition, it was not necessarily optimal:
 - it wrote '\0' twice;
 - it required to know the buffer length, that is the input string
   length + 1, in advance.

Remove this function, and let the client use strcspn() manually.