Remove check for TURBOPACK=1

Author: timneutkens

Diff for: packages/next/src/cli/next-start.ts

@@ -33,16 +33,6 @@ const nextStart = async (options: NextStartOptions, directory?: string) => {
     printAndExit(getReservedPortExplanation(port), 1)
   }
 
-  const isTurbopack = Boolean(
-    options.turbo ||
-      options.turbopack ||
-      // TODO: Used for Testing in Next.js CI. Rename to something better like `NEXT_TEST_TURBOPACK`.
-      process.env.TURBOPACK
-  )
-  if (isTurbopack) {
-    process.env.TURBOPACK = '1'
-  }
-
   await startServer({
     dir,
     isDev: false,

Diff for: packages/next/src/server/next-server.ts

@@ -192,20 +192,6 @@ export default class NextNodeServer extends BaseServer<
     this.isDev = isDev
     this.sriEnabled = Boolean(options.conf.experimental?.sri?.algorithm)
 
-    // @ts-expect-error internal field not publicly exposed
-    const isTurbopackBuild = this.nextConfig.experimental?.isTurbopackBuild
-    if (!isDev && typeof isTurbopackBuild !== 'undefined') {
-      if (process.env.TURBOPACK && !isTurbopackBuild) {
-        throw new Error(
-          `Invariant: --turbopack is set but the build used Webpack`
-        )
-      } else if (!process.env.TURBOPACK && isTurbopackBuild) {
-        throw new Error(
-          `Invariant: --turbopack is not set but the build used Turbopack. Add --turbopack to "next start".`
-        )
-      }
-    }
-
     /**
      * This sets environment variable to be used at the time of SSR by head.tsx.
      * Using this from process.env allows targeting SSR by calling