Remove dead EFP references left over from its removal

EFP (extended futility pruning) was removed from Search()'s move loop
in favor of LMP's joint FH% table, but several comments, a dead
counter (u64EFPPruned, never incremented anywhere), and two unused
defines (EFP_FH_MIN_SAMPLES/EFP_FH_PRUNE_THRESHOLD_PCT_THOUSANDTHS)
were left behind claiming EFP still gates something. Clean these up:

- Delete u64EFPPruned (COUNTERS and g_Options) and its dead
  reader/writer in root.c.
- Delete the two unused EFP_FH_* defines in search.c, and the stale
  fAnyMoveEFPPruned comment describing a branch that no longer
  skips a store.
- Fix dangling prose references to EFP in generate.c, chess.h,
  searchsup.c, and testsearch.c that describe present-tense behavior
  EFP no longer participates in, preserving the historical rationale
  where it's still relevant (e.g. why EstimateHangingMaterialCp can
  return a negative value).

Deliberately left alone (flagged for a separate decision, not a
clear delete): script.c's still-live JOINT_EXTRA_VERIFY calibration
labels ("EFP tier1/tier2"), search.c's _PostReduceDepthBucketEfpAligned
(same live mechanism, name only), searchsup.c's LMP_BASE_MOVENUM/
LMP_PER_PLY_MOVENUM dead macros (kept pending a real floor/ceiling
decision), and PLY_INFO.iPositional[2] (now write-only, but removing
it touches Eval()'s call signature everywhere).

Comment/dead-code only, no logic touched.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

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