Remove redundant tVisibility

The CI data contains a duplicated visibility file:

c0661c300babdfeb4a6af1dce18ad92c  a12-212-1sec-upgraded.vis
b2493ae33905873c3cf51dc3af1e43b3  a12-212-1sec-upgraded.vis.zip
c0661c300babdfeb4a6af1dce18ad92c  a12-SB212-1sec.vis
16c66d5affdd531938031c329f2da6a7  a12-SB212-1sec.vis.zip

tVisibility tests whether conversion of a12-SB212-1sec.vis yields the same file as a12-212-1sec-upgraded.vis. This is always the case, since vis-upgrader doesn't have to do anything. To make matters more complicated, there is some ambiguity in the different visibility formats. John uses 'legacy' and the 'current' format, while we opted for 'v1' and 'v2'. In practice, there were even some intermediate formats. Ideally, we should have tests to go from any old format to the latest format, but that is beyond the scope of this MR.

tVisibility also contains a test test_stream_processing to test the stream-aggregator but this should really be part of a separate test. Also, since only Duncan knows exactly how it works and the test doesn't seem to work anyway, it is removed for now.