fix: nil: Remove isc_mem_strndup()

The isc_mem_strndup() function had a single caller, the HTTP/2
request-path handling, which now uses isc_mem_allocate() and strlcpy()
directly.  Remove the function from the libisc API.

Closes #6087

Merge branch '6087-remove-isc_mem_strndup' into 'main'

See merge request isc-projects/bind9!12240