Remove some incorrect code that was present after since I added
support for idle timeouts in CL 22670.

This code actually caused a bug (a rare goroutine leak) rather than
prevent a bogus connection reuse.

The t.idleMu mutex already protects most the invariants, including an
explicit Stop call. There's only one Stop call on that timer, and it's
guarded by t.idleMu. What idleMu doesn't protect against is the timer
firing on its own. But we don't need code to protect against that case
because the goroutine that is created via AfterFunc when the timer
fires already checks the invariants:

  // closeConnIfStillIdle closes the connection if it's still sitting idle.
  // This is what's called by the persistConn's idleTimer, and is run in its
  // own goroutine.
  func (pc *persistConn) closeConnIfStillIdle() {
     t := pc.t
     t.idleMu.Lock()
     defer t.idleMu.Unlock()
     if _, ok := t.idleLRU.m[pc]; !ok {
       // Not idle.
       return
     }

(note the "Not idle." part).

Tested by hand with the repro code from #25621. No more leaks.

Fixes #25621

Change-Id: Idf011a4cb1fcd01f55a5a6269e4c0ee5f4446786
Reviewed-on: https://go-review.googlesource.com/123315
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>