clang-tidy: use = default

Found with modernize-use-equals-default

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/Lexer.cpp

@@ -49,11 +49,6 @@ Lexer::Lexer (const std::string& text)
 {
 }
 
-////////////////////////////////////////////////////////////////////////////////
-Lexer::~Lexer ()
-{
-}
-
 ////////////////////////////////////////////////////////////////////////////////
 // When a Lexer object is constructed with a string, this method walks through
 // the stream of low-level tokens.