[RyuJIT] Delete dead code (dotnet/coreclr#20411)

[RyuJIT] Delete dead code (dotnet/coreclr#20411)

* Delete dead code

optFindLocalInit and related functions (optIsTrackedLocal, lvaLclVarRefs, lvaLclVarRefsAccumIntoRes, lvaLclVarRefsAccum) are not used anywhere.
Also delete a bunch of undefined function declarations.

* Cleanup DataFlow callback comment

Commit migrated from https://github.com/dotnet/coreclr/commit/a0c8ff74c46cb7792796171eea9122d10a9f9bcd