Revert "Add a --webpack flag and default --turbopack to true (#84216)"

[ijjk]

This is causing a lot of our E2E deploy tests to fail so reverting to allow discussing/investigating separately

x-ref: https://github.com/vercel/next.js/actions/runs/18081410586
x-ref: #84216

This reverts commit 97056e0.

[ijjk]

Failing test suites

Commit: 32c1073 | About building and testing Next.js

pnpm test-start-experimental test/e2e/app-dir/segment-cache/search-params/segment-cache-search-params.test.ts(Experimental)

• segment cache (search params) > when fetching with PPR, does not include search params in the cache key (DD)

Expand output

● segment cache (search params) › when fetching with PPR, does not include search params in the cache key

thrown: "Exceeded timeout of 120000 ms for a test.
Add a timeout value to this test to increase the timeout, if this is a long-running test. See https://jestjs.io/docs/api#testname-fn-timeout."

  12 |   }
  13 |
> 14 |   it('when fetching with PPR, does not include search params in the cache key', async () => {
     |   ^
  15 |     let act: ReturnType<typeof createRouterAct>
  16 |     const browser = await next.browser('/search-params', {
  17 |       beforePageLoad(page) {

  at it (e2e/app-dir/segment-cache/search-params/segment-cache-search-params.test.ts:14:3)
  at Object.describe (e2e/app-dir/segment-cache/search-params/segment-cache-search-params.test.ts:5:1)

[franky47]

I also had failures with 15.6.0-canary.{33,34} in the nuqs e2e tests, this PR solved them.

Failures were flaky (couldn't reproduce locally and failed randomly in CI), but were limited to the pages router, and exacerbated with the basePath option and/or the use of the React Compiler.

Failed runs:

• 15.6.0-canary.34
• 15.6.0-canary.33