chore: remove unnecessary workaround

It is not needed anymore after we fixed the `app` parser

src/Init/Lean/Parser/Term.lean

@@ -116,7 +116,7 @@ def doExpr := parser! termParser
 def doElem := doLet <|> doId <|> doPat <|> doExpr
 def doSeq  := sepBy1 doElem "; "
 def bracketedDoSeq := parser! "{" >> doSeq >> "}"
-@[builtinTermParser] def liftMethod := parser! checkRBPGreater (appPrec-1) "expected parentheses monad lift operator" >> leftArrow >> termParser
+@[builtinTermParser] def liftMethod := parser! leftArrow >> termParser
 @[builtinTermParser] def «do»  := parser! "do " >> (bracketedDoSeq <|> doSeq)
 
 @[builtinTermParser] def not    := parser! symbol "¬" 40 >> termParser 40

tests/lean/run/LiftMethodIssue.lean

@@ -3,3 +3,7 @@ new_frontend
 def tst : IO (Option Nat) := do
 x? : Option Nat ← pure none;
 pure x?
+
+def tst2 (x : Nat) : IO (Option Nat) := do
+x? : Option Nat ← pure x;
+if x?.isNone then pure (x+1) else pure x?