[otbn] Remove outdated comment from RTL
[otbn] Remove outdated comment from RTL

The LFSR is now in otbn_rnd.sv, the placeholder is no longer needed.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

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