RfhToIoThreadClientMap: Remove reference to WEBVIEW_PRERENDER2 feature flag

Before this CL, `RfhToIoThreadClientMap::RenderFrameHostChanged` logic
to handle prerender activation was guarded undef rlag.

This CL removes the flag guard, since we are moving away from the
WEBVIEW_PRERENDER2 feature flag.

The change is safe, since the logic is safe to be applied to non-Prerender2
FrameTree swaps.

The author tried to keep the if condition, but it seems difficult to do so:
- We can't rely on `new_rfh->GetLifecycleState()` check, since both RFHs
  is in state `kActive` on Prerender2 activation.
- `WebContentsDelegate::IsPrerender2Supported()` is not reliable, since
  Prerender2 allowance condition may have changed (TOCTOU).

Bug: 339561855
TEST: android_webview/javatests/src/org/chromium/android_webview/test/AwPrerenderTest.java
Change-Id: I60ceac0f97232188ee24294e753fc9ee3868decf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5527170
Reviewed-by: Richard (Torne) Coles <torne@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300433}