[dv] Comment out example build modes from common_sim_cfg.hjson
[dv] Comment out example build modes from common_sim_cfg.hjson

These are supposed to be documentation, but were actually defining
bogus build modes (I noticed by running dvsim.py --list).

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan