prior bounds: make sure that lower and upper bounds only affect mode-finding

https://github.com/DynareTeam/dynare/commit/99dbc8c74d0ecf4c7a610c4a394fd85028cf2741 introduced a bug by disabling the resetting of the prior bounds used for the MCMC. The manual clearly states the bounds are only operational during mode-finding, but not during MCMC (and therefore prior sampling)
(cherry picked from commit 127a543b)