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.