minor: delete duplicate paragraph about $abs and null/NaN

Author: ksuarz

source/reference/operator/aggregation/abs.txt

@@ -34,10 +34,6 @@ Behavior
 
 .. include:: /includes/extracts/agg-expression-null-operand-abs.rst
 
-If the argument resolves to a value of ``null`` or refers to a field
-that is missing, :expression:`$abs` returns ``null``. If the
-argument resolves to ``NaN``, :expression:`$abs` returns ``NaN``.
-
 .. list-table::
    :header-rows: 1
    :widths: 85 15