Revert "Add String.length_join and List.length_join (#770)"

This reverts commit 123fb93.

Std/Data/List/Lemmas.lean

@@ -365,9 +365,6 @@ theorem exists_of_mem_join : a ∈ join L → ∃ l, l ∈ L ∧ a ∈ l := mem_
 
 theorem mem_join_of_mem (lL : l ∈ L) (al : a ∈ l) : a ∈ join L := mem_join.2 ⟨l, lL, al⟩
 
-theorem length_join (l : List (List α)) : (join l).length = Nat.sum (l.map length) := by
-  induction l <;> simp [*]
-
 /-! ### bind -/
 
 theorem mem_bind {f : α → List β} {b} {l : List α} : b ∈ l.bind f ↔ ∃ a, a ∈ l ∧ b ∈ f a := by

Std/Data/String/Lemmas.lean

@@ -516,11 +516,6 @@ theorem join_eq (ss : List String) : join ss = ⟨(ss.map data).join⟩ := go ss
 @[simp] theorem data_join (ss : List String) : (join ss).data = (ss.map data).join := by
   rw [join_eq]
 
-@[simp] theorem length_data (s : String) : s.data.length = s.length := rfl
-
-theorem length_join (l : List String) : (join l).length = Nat.sum (l.map length) := by
-  simp [length, List.length_join, List.map_map, (funext length_data : List.length ∘ data = length)]
-
 theorem singleton_eq (c : Char) : singleton c = ⟨[c]⟩ := rfl
 
 @[simp] theorem data_singleton (c : Char) : (singleton c).data = [c] := rfl