[llvm-cov] Remove "default:" label in the switch covering all enum values.

Summary:
Fixing the build breakage:
http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/27309

Reviewers: vsk, allevato, Dor1s

Reviewed By: Dor1s

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54323

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346514 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-cov/CodeCoverage.cpp

@@ -1039,8 +1039,6 @@ int CodeCoverageTool::doExport(int argc, const char **argv,
     Exporter = llvm::make_unique<CoverageExporterLcov>(*Coverage.get(),
                                                        ViewOpts, outs());
     break;
-  default:
-    llvm_unreachable("Unknown coverage output format!");
   }
 
   if (SourceFiles.empty())