Unit: removed a useless assignment.

As was pointed out by the cppcheck[0] static code analysis utility there
was a useless assignment in nxt_unit_request_read(). The size parameter
is passed in by value and was being modified without being used again.

[0]: https://cppcheck.sourceforge.io/

src/nxt_unit.c

@@ -3076,7 +3076,6 @@ nxt_unit_request_read(nxt_unit_request_info_t *req, void *dst, size_t size)
         }
 
         req->content_length -= res;
-        size -= res;
 
         dst = nxt_pointer_to(dst, res);