Remove unused code in a libc++ test.

Other tests in this directory use this type, so it's probably copypasta from
there.

(test_buf only forwards to the superclass in all tests where it's used though,
so I wonder if it can be replaced with just using filebuf / wfilebuf
everywhere?)


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@210019 91177308-0d34-0410-b5e6-96231b3b80d8