Revert "Enabled parallel backend on panda_bench_xilinx_vvd.sh."

This reverts commit fe1dabc.

Author: fabrizioferrandi

.github/workflows/minimal.yml

@@ -887,7 +887,6 @@ jobs:
           - test: xilinx_ise
             datapoints: CYCLES,PERIOD,AREA,REGISTERS,DSPS,BRAMS
           - test: xilinx_vvd
-            args: -c=--parallel-backend
             datapoints: CYCLES,PERIOD,AREA,REGISTERS,DSPS,BRAMS
     env:
       outname: panda_bench_${{matrix.test}}