public FieldValue removeFieldValue(String fieldName) {
        Field field = getField(fieldName);
        if (field == null) {
            return null;
        }
        return removeFieldValue(field);
    }
Convenience method to remove the value of a named field. This is the same as calling {@link #getField(String)},
and using the returned value to call {@link #removeFieldValue(Field)}. If the named field does not exist, this
method returns null.

@param fieldName The name of the field whose value to remove.
@return The previous value of the field, or null.
document/src/main/java/com/yahoo/document/datatypes/StructuredFieldValue.java