[otbn] Remove the OpenSection logic in the RIG's Program class
[otbn] Remove the OpenSection logic in the RIG's Program class

This was supposed to be useful for adding instructions to the "current
section", but we don't actually use it, so it's probably better to
ditch it.

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

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