SoilDyn: Remove flag SlDNonLinearForcePortionOnly

[RBergua]

This PR is ready to be merged.

Feature or improvement description
As noted in PR #3319, the SlDNonLinearForcePortionOnly flag was previously hardcoded to TRUE and had no functional effect. This PR removes that legacy flag from the SoilDyn code to improve clarity.

The updated logic is straightforward:

1. The 6x6 linear stiffness matrix defined in SoilDyn (e.g., CalcOption = 1 or 3) is always passed to and included in SubDyn.
2. Only nonlinear loads are transferred from SoilDyn to SubDyn.

The SlDNonLinear flag is now exclusively used when CalcOption = 3 (REDWIN) and it affects only reaction load outputs in SubDyn (sensor-written values, not physical behavior).

Impacted areas of the software
SoilDyn.

Test results, if applicable
No changes expected.

[andrew-platt]

I see we have a SoilDyn driver, but no test cases for it. I'll look through my development test cases and see if there is one I can repurpose for testing here. I'll add it as a separate PR if I find it.