chore: remove unused syntax (leanprover#8760)

Removes unused `tailrecursion` syntax.

Author: cppio

src/Lean/Elab/PreDefinition/TerminationHint.lean

@@ -152,12 +152,10 @@ def elabTerminationHints {m} [Monad m] [MonadError m] (stx : TSyntax ``suffix) :
       | _ => pure none
       else pure none
     let terminationBy? : Option TerminationBy ← if let some t := t? then match t with
-      | `(terminationBy|termination_by partialFixpointursion) =>
-        pure (some {ref := t, structural := false, vars := #[], body := ⟨.missing⟩ : TerminationBy})
       | `(terminationBy|termination_by $[structural%$s]? => $_body) =>
         throwErrorAt t "no extra parameters bounds, please omit the `=>`"
       | `(terminationBy|termination_by $[structural%$s]? $vars* => $body) =>
-        pure (some {ref := t, structural := s.isSome, vars, body})
+        pure (some {ref := t, structural := s.isSome, vars, body : TerminationBy})
       | `(terminationBy|termination_by $[structural%$s]? $body:term) =>
         pure (some {ref := t, structural := s.isSome, vars := #[], body})
       | `(terminationBy?|termination_by?) => pure none

src/Lean/Parser/Term.lean

@@ -720,11 +720,10 @@ the inferrred termination measure will be suggested.
 
 -/
 @[builtin_doc] def terminationBy := leading_parser
-  "termination_by " >> (
-  (nonReservedSymbol "tailrecursion") <|>
-  (optional (nonReservedSymbol "structural ") >>
-   optional (atomic (many (ppSpace >> Term.binderIdent) >> " => ")) >>
-   termParser))
+  "termination_by " >>
+  optional (nonReservedSymbol "structural ") >>
+  optional (atomic (many (ppSpace >> Term.binderIdent) >> " => ")) >>
+  termParser
 
 @[inherit_doc terminationBy, builtin_doc]
 def terminationBy? := leading_parser