[DWARFYAML] Remove 'default' tag. NFC.

This patch is trying to make build bots happy.

Failed bots:
http://lab.llvm.org:8011/builders/ppc64le-lld-multistage-test/builds/10705
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-android/builds/33595

llvm/lib/ObjectYAML/DWARFEmitter.cpp

@@ -510,8 +510,6 @@ writeRnglistEntry(raw_ostream &OS, const DWARFYAML::RnglistEntry &Entry,
       return std::move(Err);
     encodeULEB128(Entry.Values[1], OS);
     break;
-  default:
-    llvm_unreachable("unrecognized range list encoding");
   }
 
   return OS.tell() - BeginOffset;