Don't compute parameters twice (in a row)

Removed a redundant call to compute parameters.  This function is called
from within the last function call, no need to call it again.

backend/avision.c

@@ -8140,7 +8140,6 @@ sane_start (SANE_Handle handle)
   if (status != SANE_STATUS_GOOD) {
     return status;
   }
-  compute_parameters (s);
   
   /* for non ADF scans (e.g. scanimage --batch-prompt on a Flatbed
      scanner) make sure we do not assume it's an ADF scan and