HttpStreamPool: Remove redundant pending job check in ProcessPendingJob

In `HttpStreamPool::AttemptManager::ProcessPendingJob`, we checked for
the number of pending request jobs and preconnects before proceeding.
However, this check was redundant.

The subsequent call to `MaybeAttemptTcpBased()` eventually calls
`CanAttemptConnection()`, which performs the exact same check for
pending jobs. If there are no pending jobs, `CanAttemptConnection()`
returns `kNoPendingJob`, and no new connection attempt is made.

This change removes the unnecessary check from `ProcessPendingJob()` to
simplify the code and eliminate redundancy, relying on the existing
check within `CanAttemptConnection()`.

Co-authored by Gemini.

#ai-assisted

Bug: 383606724
Change-Id: I8027aa6b641578276227d22f54d46567e8b91ae2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6849306
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1501662}