Remove unused arg field

src/interpreter.cc

@@ -2596,9 +2596,6 @@ void _interpret(Program* program, int a2)
             programFatalError(err);
         }
 
-        // Reset argument counter for error reporting.
-        program->arg = 0;
-
         handler(program);
     }
 

src/interpreter.h

@@ -171,7 +171,6 @@ typedef struct Program {
     bool exited;
     ProgramStack* stackValues;
     ProgramStack* returnStackValues;
-    int arg;
 } Program;
 
 typedef void OpcodeHandler(Program* program);