simplify tests where Identity() was needed as last transformation in a sequence

[LucaMarconato]

In a test for sequence transformations I had to add some Identity() transformations due to a limitation of _get_current_output_axes(). See here:

spatialdata/tests/transformations/test_transformations.py

Line 435 in a9fb477

# the function _get_current_output_axes() doesn't get called for the last transformation in a sequence,

• verify that this is not needed anymore and drop the Identity() from the tests

[LucaMarconato]

I verified, the test is correct also when the last instances of Identity() are dropped, as expected. We need to make a small PR to remove them from the test.