Remove previous LVS results before making lvs-* and lvs-gds-* targets.

[d-m-bailey]

This fixes an issue on caravel_user_project where previous correct lef lvs results were used when gds lvs failed.

caravel_user_project issue

This has been tested by copying caravel/openlane/digital_pll to openlane/digital_pll and copying digital_pll.v, digital_pll_controller.v, and ring_osc2x13.v from caravel/verilog/rtl to verilog/rtl/. Use netgen 1.5.228

Before change

make  digital_pll
make lvs-digital_pll
(lef lvs passes)
rm gds/digital_pll.gds*
touch gds/digital_pll.gds
make lvs-gds-digital_pll
(gds lvs fails, but finishes without errors)

After change

make  digital_pll
make lvs-digital_pll
(lef lvs passes)
rm gds/digital_pll.gds*
touch gds/digital_pll.gds
make lvs-gds-digital_pll
(make fails)

[RTimothyEdwards]

Looks reasonable, although it looks like all of these Makefile recipes are leaving behind "tmp" directories without cleaning them up afterward. Makes more sense to remove them at the end of each recipe (as well as at the beginning, as a precaution).