Blink's SpeculationRulesPrefetchProxy runtime feature flas is gone.

We originally disabled it for brave/brave-browser#23433 via
#13772, but the issue is no longer reproducible in
cr121, so we can just drop the call to disable the flag.

Chromium change:

https://chromium.googlesource.com/chromium/src/+/d1cf902be6fc9ae8654fe5a6c466dfb51f782197

commit d1cf902be6fc9ae8654fe5a6c466dfb51f782197
Author: Jeremy Roman <jbroman@chromium.org>
Date:   Thu Nov 9 16:38:02 2023 +0000

    Remove RuntimeEnabledFeatures checks for SpeculationRules and SpeculationRulesPrefetchProxy.

    These have been shipped for almost a year.

    Bug: 1173646

renderer/brave_content_renderer_client.cc

@@ -102,8 +102,6 @@ void BraveContentRendererClient::
   if (!base::FeatureList::IsEnabled(blink::features::kBraveWebSerialAPI)) {
     blink::WebRuntimeFeatures::EnableFeatureFromString("Serial", false);
   }
-  blink::WebRuntimeFeatures::EnableFeatureFromString(
-      "SpeculationRulesPrefetchProxy", false);
   blink::WebRuntimeFeatures::EnableFeatureFromString("AdTagging", false);
 }