refactor(simplex): remove unused pivot bindings

[rasheedja]

Summary

• Remove the unused newEnteringVarTerm binding and unused alternative row-update function from pivoting.
• Rename the active f2 helper to updateRow so its purpose is clear.
• Preserve the active pivot calculations and public API.

This cleanup targets master independently; it does not require the correctness fixes to merge first. The map-fold and printer cleanups are already included in #18 and #19.

Validation

• stack test --no-terminal: 271 examples, 0 failures on this branch.
• cabal check, Fourmolu 0.15.0.0, git diff --check, and stack haddock --no-haddock-deps passed.
• All five cleanup branches merge cleanly with each other and fixes fix(simplex): handle phase-one variables and constant constraints #17–fix(prettify): render fractions and separators correctly #19 in a temporary integration worktree.
• Combined integration suite: 288 examples, 0 failures.