bpo-45208: Make test_pdb.test_checkline_is_not_executable() quiet (GH-28354) (GH-28363)

test_pdb.test_checkline_is_not_executable() no longer writes output
to stdout.

Remove also unused variables 'f'.
(cherry picked from commit e08e491a6ceea8ca105612df10147418c4e105b8)

Co-authored-by: Victor Stinner <vstinner@python.org>