[test] two small cleanups:

* Remove unused type from is_assignable.pass.cpp

* Don't specialize `common_type<::X<float>>` in common_type.pass.cpp, which violates the requirements of [meta.trans.other]/5

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@336618 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed