Remove stale non-public sparsity bindings from docs

[ChrisRackauckas-Claude]

Please ignore this draft until it has been reviewed by @ChrisRackauckas.

What changed

Remove ModelingToolkitBase.jacobian_sparsity and ModelingToolkitBase.hessian_sparsity from the public code-generation @docs block.

ModelingToolkitBase no longer binds these names after #4989. The underlying Symbolics names are neither exported nor declared public, so switching the docs to Symbolics.* would preserve a non-public dependency. This patch removes only the two invalid entries.

Failing before the fix

The full CI-equivalent LTS docs command was run on clean current master 38d91d81dcd77319a70abb93882ba6da13510600:

timeout 7200 env JULIA_DEBUG=Documenter DISPLAY=:0 \
  xvfb-run -a -s '-screen 0 1024x768x24' \
  julia +lts --project=docs/ --startup-file=no docs/make.jl

It exited 1 and reported:

Error: undefined binding `ModelingToolkitBase.hessian_sparsity` in @docs block
docs/src/API/codegen.md:36-53

That run also encountered a transient HTTP 429 from the external ColPrac link. A direct retry returned HTTP 200; no link check was disabled.

A source-history bisect identifies 63f684c as the binding boundary. The same owner-migration fixed sibling test references in #4991, but the docs entry remained.

Passing after the fix

The identical full LTS docs command exited 0:

linkcheck 'https://github.com/SciML/ColPrac/blob/master/README.md' status: 200
[ Info: RenderDocument: rendering document
exit 0

Additional local gates:

GROUP=QA julia +1.12 --project=. -e 'using Pkg; Pkg.test()'
julia +1.12 --startup-file=no -m Runic -c .
typos docs/src/API/codegen.md
git diff --check

QA | 52/52
Testing ModelingToolkit tests passed
Runic: exit 0
typos: exit 0
git diff --check: exit 0

The diff is two deletions in one docs file. No source, dependency, or public API declaration changes. Julia 1/pre test groups were not run locally; the owning docs gate was run on LTS to match CI.

[ChrisRackauckas-Claude]

Superseded by #5003, which now contains the stale API binding removal together with the typo-only cleanup.