prescribed_bcs_enabled is dead: reads removed in #121, declaration and docs remain

`Model%prescribed_bcs_enabled` (`src/SELF_Model.f90:107`) is read nowhere in `src/`, in any of the CPU, `gpu`, or `apu` variants.

Its three reads — one each in `SELF_DGModel1D_t`, `SELF_DGModel2D_t`, `SELF_DGModel3D_t` — were removed by 3fa444f (#121, "Add extensible boundary condition system") when the boundary-condition registry replaced hardcoded BC dispatch. The declaration was not.

It is still written in ~40 files under `examples/` and `test/`, and still documented as a working performance knob:

- `docs/Models/linear-euler-2d-model.md:231`
- `docs/Models/linear-euler-3d-model.md:219`

Both say setting it `.false.` "can improve the time-to-solution for your simulation by avoiding unnecessary host-device memory movement." It has no effect.

Downstream cost: a user writing a prescribed BC against the registry finds a flag named for exactly that feature, defaulting `.true.`, documented as live, and reasonably concludes it gates their BC.

Suggested: remove the declaration and the `examples/`/`test/` assignments, and drop the two doc paragraphs. If it is instead meant to come back as a registry-aware optimisation, a comment on the declaration saying so would be enough.

Start at the declaration in src/SELF_Model.f90:107, then search the examples/ and test/ trees for assignments to prescribed_bcs_enabled. Remove the dead declaration and assignments, and remove the two documented performance-knob paragraphs in docs/Models/linear-euler-2d-model.md:231 and docs/Models/linear-euler-3d-model.md:219. Done means no remaining declaration, assignments, or claims that the flag is active.

Written by the indexing model from the issue text.