Remove the |NIP_CURR_POS| bytecode instruction

After the previous commit is is not used anywhere.

lib/compiler/opcodes.js

@@ -7,7 +7,6 @@ module.exports = {
   POP_CURR_POS:     3,    // POP_CURR_POS
   POP_N:            4,    // POP_N n
   NIP:              5,    // NIP
-  NIP_CURR_POS:     6,    // NIP_CURR_POS
   APPEND:           7,    // APPEND
   WRAP:             8,    // WRAP n
   TEXT:             9,    // TEXT

lib/compiler/passes/generate-bytecode.js

@@ -35,12 +35,6 @@ var utils = require("../../utils"),
  *        stack.pop();
  *        stack.push(value);
  *
- *  [6] NIP_CURR_POS
- *
- *        value = stack.pop();
- *        currPos = stack.pop();
- *        stack.push(value);
- *
  *  [7] APPEND
  *
  *        value = stack.pop();

lib/compiler/passes/generate-javascript.js

@@ -197,11 +197,6 @@ module.exports = function(ast, options) {
       '          ip++;',
       '          break;',
       '',
-      '        case ' + op.NIP_CURR_POS + ':',     // NIP_CURR_POS
-      '          peg$currPos = stack.splice(-2, 1)[0];',
-      '          ip++;',
-      '          break;',
-      '',
       '        case ' + op.APPEND + ':',           // APPEND
       '          stack[stack.length - 2].push(stack.pop());',
       '          ip++;',
@@ -482,13 +477,6 @@ module.exports = function(ast, options) {
             ip++;
             break;
 
-          case op.NIP_CURR_POS:     // NIP_CURR_POS
-            value = stack.pop();
-            parts.push('peg$currPos = ' + stack.pop() + ';');
-            parts.push(stack.push(value));
-            ip++;
-            break;
-
           case op.APPEND:           // APPEND
             value = stack.pop();
             parts.push(stack.top() + '.push(' + value + ');');