Change OpenTitan to no-assert to override Verilator's change-of-default

[wsnyder]

Verilator now has --assert the default, so get back the OpenTitan loss due to this.

[gezalore]

Have you investigated why --assert causes the drop? as there are no active assert/assume statements in the OpenTitan code (all are ifdefed out), I presume it is because of all the unique case statements?

Note Vortex has a drop which I suspect is due to the same. (It also had a broken assertion which caused some run failures. I fixed that I have fixed here 033e7ef.)

I'm leaning towards keeping the new default if the effect is well understood (and maybe improve speed later), or at least all designs should use either --assert or --no-assert.

[wsnyder]

Ok, let's keep --assert. I haven't debugger the drop, but agree suspect case related.