[Parse] Remove dead code (#15723)

LeadingTrivia.empty() does nothing.
It was meant to be clear(), but clear() is also meaningless because
'LeadingTrivia' is immediately overwritten.

Author: rintaro

lib/Parse/Parser.cpp

@@ -559,7 +559,6 @@ void Parser::markSplitToken(tok Kind, StringRef Txt) {
   SplitTokens.back().setToken(Kind, Txt);
   Trivia EmptyTrivia;
   SyntaxContext->addToken(SplitTokens.back(), LeadingTrivia, EmptyTrivia);
-  LeadingTrivia.empty();
   TokReceiver->receive(SplitTokens.back());
 }