Remove outdated logic from HttpStreamPool::AttemptManager

- We abandoned the idea of having "paused jobs", so Preconnect()
  should be called only when the active stream count is less than
  requested.
- RequiresHTTP11() isn't used any longer.

Bug: N/A
Change-Id: I6c93e650b3e3171c1e346dface5c8dccabbeaad2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6627095
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Nidhi Jaju <nidhijaju@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1471600}