remove Symbol::empty

the only use of this function is to determine whether a lambda has a non-set
formal, but this use is arguably better served by Symbol::set and using a
non-Symbol instead of an empty symbol in the parser when no such formal is present.

src/libexpr/symbol-table.hh

@@ -60,11 +60,6 @@ public:
         return s;
     }
 
-    bool empty() const
-    {
-        return s->empty();
-    }
-
     friend std::ostream & operator << (std::ostream & str, const Symbol & sym);
 };