Remove dead axisymmetric Jacobian terms from the SST source - #2986

Title: Remove dead axisymmetric Jacobian terms from the SST source

Proposed Changes

In CSourcePieceWise_TurbSST::ComputeResidual, ResidualAxisymmetricConvectionDiffusion subtracts
yinv * Volume * rhov from the diagonal of Jacobian_i, but the "Implicit part" right after it assigns
Jacobian_i[i][j] = ..., so those contributions were always discarded. This PR removes the dead lines
and adds a comment explaining why the term is intentionally left out of the Jacobian.

I first tried keeping the terms (calling the axisymmetric function after the implicit block), but:

• As written, the term is not the derivative w.r.t. the conservative variables (rho k, rho w); that would
  be yinv * Volume * v, not yinv * Volume * rho * v.
• Either way, it reduces diagonal dominance for flow towards the axis (v < 0), where yinv is large.

Results on axisymmetric_rans/air_nozzle/air_nozzle_restart.cfg (serial, until rms[Rho] = -12):

So the current behaviour is the right one, and this PR only makes it explicit. Results are bit-identical
(the removed lines never had an effect), so no regression values change. I checked
axi_rans_air_nozzle_restart (serial and 2 ranks) and axi_rans_air_nozzle_species locally.

Related Work

#2948 edits the same implicit block of the SST source; this PR does not touch those lines.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.