Drop unneccessary default case from switch introduced in r219809

This silences:
../tools/clang/tools/libclang/CIndex.cpp:6451:3: warning: default label
in switch which covers all enumeration values [-Wcovered-switch-default]

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219818 91177308-0d34-0410-b5e6-96231b3b80d8