Fix-forward -Wcovered-switch-default error in #134429

Remove the default case to avoid buildbot error (https://lab.llvm.org/buildbot/#/builders/66/builds/12382/steps/8/logs/stdio):
```
/home/b/sanitizer-x86_64-linux/build/llvm-project/llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.cpp:85:3: error: default label in switch which covers all enumeration values [-Werror,-Wcovered-switch-default]
   85 |   default:
      |   ^
```

(also https://llvm.org/docs/CodingStandards.html#don-t-use-default-labels-in-fully-covered-switches-over-enumerations)

Author: thurstond

llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.cpp

@@ -82,9 +82,6 @@ storageClassRequiresExplictLayout(SPIRV::StorageClass::StorageClass SC) {
   case SPIRV::StorageClass::DeviceOnlyINTEL:
   case SPIRV::StorageClass::HostOnlyINTEL:
     return false;
-  default:
-    llvm_unreachable("Unknown storage class");
-    return false;
   }
 }