Fix: bt2: unused assignments in test_field_class.py

Fix these:

    ./tests/bindings/python/bt2/test_field_class.py:101:9: F841 local variable 'field' is assigned to but never used
    ./tests/bindings/python/bt2/test_field_class.py:124:9: F841 local variable 'field' is assigned to but never used
    ./tests/bindings/python/bt2/test_field_class.py:215:9: F841 local variable 'field' is assigned to but never used
    ./tests/bindings/python/bt2/test_field_class.py:234:9: F841 local variable 'field' is assigned to but never used
    ./tests/bindings/python/bt2/test_field_class.py:251:9: F841 local variable 'field' is assigned to but never used
    ./tests/bindings/python/bt2/test_field_class.py:402:9: F841 local variable 'field' is assigned to but never used
    ./tests/bindings/python/bt2/test_field_class.py:428:9: F841 local variable 'field' is assigned to but never used
    ./tests/bindings/python/bt2/test_field_class.py:445:9: F841 local variable 'field' is assigned to but never used
    ./tests/bindings/python/bt2/test_field_class.py:1183:9: F841 local variable 'field' is assigned to but never used
    ./tests/bindings/python/bt2/test_field_class.py:1222:9: F841 local variable 'field' is assigned to but never used

In all these cases, the assignment has no effect.  The original intent
was probably to do `field.value = ...`, so use that everywhere.

Reported-by: flake8
Change-Id: If28fab3d1a8facb0123ab3a2dc0b83062ebed38e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2089