[otbn,dv] Remove unused rnd_req field
[otbn,dv] Remove unused rnd_req field

This was added in 051133d, but isn't needed: that commit eventually
switched to tracking the EDN request in a RND_REQ external register.

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

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