Revert "refactor: tell gyp to use c++17 when building with our node"

This reverts commit 41a03a5799.

script/spec-runner.js

@@ -230,13 +230,8 @@ async function runMainProcessElectronTests () {
 }
 
 async function installSpecModules (dir) {
-  // v8 headers use c++17 so override the gyp default of -std=c++14,
-  // but don't clobber any other CXXFLAGS that were passed into spec-runner.js
-  const CXXFLAGS = ['-std=c++17', process.env.CXXFLAGS].filter(x => !!x).join(' ');
-
   const nodeDir = path.resolve(BASE, `out/${utils.getOutDir({ shouldLog: true })}/gen/node_headers`);
   const env = Object.assign({}, process.env, {
-    CXXFLAGS,
     npm_config_nodedir: nodeDir,
     npm_config_msvs_version: '2019',
     npm_config_yes: 'true'