Refactor: Remove obsolete cg_in_lcao code

[AsTonyshment]

What's changed?

ks_solver=cg_in_lcao was introduced in #3473 but was later removed in #5257. There are still some dead code, which is rather misleading. This PR aims to remove these obsolete cg_in_lcao code.

• Reject the unsupported ks_solver=cg_in_lcao value during INPUT validation.
• Remove its stale matrix-layout, SCF-label, density-matrix, and iterative-diagonalization branches.
• Remove the unused LCAO CG thresholds and HSolverLCAO device template parameter left by the old implementation.
• Restore the MPI/serial separation of density-matrix multiplication helpers.

User-visible behavior

LCAO inputs using cg_in_lcao now fail during input validation instead of reaching unsupported or unsafe downstream paths. Supported LCAO solvers are unaffected. The generated parameter documentation is unchanged because this value was never documented as supported.

[mohanchen]

We may need CG iteration solvers for LCAO codes in future.

[AsTonyshment]

We may need CG iteration solvers for LCAO codes in future.

Yes, we can add related code back in the future when it's fully ready. As for now, I found it sometimes very misleading, especially for coding agents (they always tell me that cg_in_lcao is not considered, while I found that this method is not usable at all).