refactor: tame down dead let eliminator in lambda RC

[hargoniX]

This PR reduces the aggressiveness of the dead let eliminator from lambda RC.

The motivation for this is that all other passes in lambda RC respect impurity but the dead let eliminator still operates under the assumption of purity. There is a couple of motivations for the elim dead let elaborator:

• unused projections introduced by the ToIR translation
• the elim dead branch pass introducing new opportunities
• closed term extraction introducing new opportunities

[hargoniX]

!bench

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase dfd3d185300df044fa087d3fa166cb6fb3bd8269 --onto 7d55c033e1f0ad8cb7c2b9ab72e5db40ce192222. You can force Mathlib CI using the force-mathlib-ci label. (2025-09-30 18:43:33)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase dfd3d185300df044fa087d3fa166cb6fb3bd8269 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-09-30 18:43:34)

[leanprover-bot]

Here are the benchmark results for commit 545fff9.
There were significant changes against commit dfd3d18:

  Benchmark   Metric           Change
  ============================================
- stdlib      grind linarith    19.1% (26.0 σ)