check_model.m: fix logic of check for non-contemporaneous variables and remove redundant checks

Closes Dynare/dynare#1792
9 jobs for check_model in 83 minutes and 6 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed build_binaries #31776

00:11:26

passed build_doc #31777

00:01:21

 
  Test
passed test_dynare++ #31781

00:16:12

passed test_matlab #31778

01:10:23

manual test_octave #31780
allowed to fail manual
manual test_old_matlab #31779
allowed to fail manual
 
  Pkg
passed pkg_macOS #31784
macOS

00:20:43

passed pkg_source #31782

00:02:14

passed pkg_windows #31783

00:18:36