[Coverage] Fix a warning

This patch fixes:

  llvm/lib/ProfileData/Coverage/CoverageMapping.cpp:219:5: error:
  default label in switch which covers all enumeration values
  [-Werror,-Wcovered-switch-default]

Author: kazutakahirata

llvm/lib/ProfileData/Coverage/CoverageMapping.cpp

@@ -216,8 +216,6 @@ Expected<int64_t> CounterMappingContext::evaluate(const Counter &C) const {
       }
       break;
     }
-    default:
-      llvm_unreachable("Unhandled CounterKind");
     }
   }