Remove some pointless forward declarations of std::complex.

According to C++ 17.6.4.2.1 these are undefined, and after r204082 they
will not compile against libc++ with clang.

git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@204701 91177308-0d34-0410-b5e6-96231b3b80d8