[compiler] Remove unreachable code

This should be the last remaining thing that fails the
-Wunreachable-code-aggressive warning.

Currently V8 DEPS update is failing due to the update containing
https://crrev.com/c/3115354, with this, the update should then be fine.

Bug: chromium:1066980
Change-Id: Ic74b3a95f90204333b0724d30463c6953047e5f7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3119999
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Auto-Submit: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Owners-Override: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76496}

src/compiler/effect-control-linearizer.cc

@@ -4246,12 +4246,10 @@ Node* EffectControlLinearizer::LowerStringToUpperCaseIntl(Node* node) {
 
 Node* EffectControlLinearizer::LowerStringToLowerCaseIntl(Node* node) {
   UNREACHABLE();
-  return nullptr;
 }
 
 Node* EffectControlLinearizer::LowerStringToUpperCaseIntl(Node* node) {
   UNREACHABLE();
-  return nullptr;
 }
 
 #endif  // V8_INTL_SUPPORT