fix: remove @[grind →] from getElem_of_getElem? (#12821)

This PR removes the `@[grind →]` attribute from
`List.getElem_of_getElem?` and `Vector.getElem_of_getElem?`. These were
identified as problematic in Mathlib by
https://github.com/leanprover/lean4/issues/12805.

🤖 Prepared with Claude Code

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/Init/Data/List/Lemmas.lean

@@ -236,7 +236,6 @@ theorem getElem?_eq_some_iff {l : List α} : l[i]? = some a ↔ ∃ h : i < l.le
     · match i, h with
       | i + 1, h => simp [getElem?_eq_some_iff, Nat.succ_lt_succ_iff]
 
-@[grind →]
 theorem getElem_of_getElem? {l : List α} : l[i]? = some a → ∃ h : i < l.length, l[i] = a :=
   getElem?_eq_some_iff.mp
 

src/Init/Data/Vector/Lemmas.lean

@@ -862,7 +862,6 @@ grind_pattern Vector.getElem?_eq_none => xs[i]? where
 theorem getElem?_eq_some_iff {xs : Vector α n} : xs[i]? = some b ↔ ∃ h : i < n, xs[i] = b :=
   _root_.getElem?_eq_some_iff
 
-@[grind →]
 theorem getElem_of_getElem? {xs : Vector α n} : xs[i]? = some a → ∃ h : i < n, xs[i] = a :=
   getElem?_eq_some_iff.mp