BKE_attribute_math: DefaultMixer: remove asserts for negative weight

The attribute smoothing node asks for the ability to have a factor outside the range of 0 and 1.
The problem with this is that there is a negative weight assertion for some of the mixers. If mixing
between 0 and 1, then at a factor of 2, one of the mixing elements will be negative.

Example:
(A,B,C - actual values, a,b,c - previous one)
A -- B -- C

Factor = 3.0f;
A = a * f + b * (1 - f) // Subtract factor from number of neighbors to invert
B = b * (f * 2) + a * (2 - f ) + c * (2 - f ) // Multiply by number of neighbors to bypass division for normalization
C = c * f + b * (1 - f)

This does not break the logic of work, but it makes it possible to use mixers incorrectly.
If at the end of the accumulation the amount is 0, then this will be the default value (and not infinity).
But this can only be obtained as a result of very misuse. For example, the smoothing node always finalizes a unit weight.