Manual saving mode is broken

Following up of bliss/bliss#3491 (closed)

A regression introduced in !222 (merged) breaks the manual saving mode used in the Bliss autofilter tests. The additional test to check whether a Stream is ready to write a frame is invalid when using the Manual saving mode.

The test tests/nexus_writer/test_nxw_timescan.py::test_nxw_timescan.py, that failed previously in bliss/bliss#3477 (closed), pass without this test, so one option is just to remove it.