Remove kstate in dyn_second_order_solver

kstate is not needed anymore as all information is found in M_.lead_lag_incidence (See #1653 (closed))

As a starting point I removed it from dyn_second_order_solver.m and tidied up the code a bit. The the functionality remains unaffected.

merged

mentioned in commit 5d2a077a

mentioned in commit wmutschl/dynare@1ea93fee

mentioned in merge request !1683 (merged)

mentioned in commit 5a8c2067