[llvm-exegesis] Remove unneeded std::move()

Caught by buildbot breakage:

/home/docker/worker_env/ppc64le-clang-rhel-test/clang-ppc64le-rhel/llvm/llvm/tools/llvm-exegesis/lib/Mips/Target.cpp:89:12: error: moving a local object in a return statement prevents copy elision [-Werror,-Wpessimizing-move]
    return std::move(Instructions);
           ^
/home/docker/worker_env/ppc64le-clang-rhel-test/clang-ppc64le-rhel/llvm/llvm/tools/llvm-exegesis/lib/Mips/Target.cpp:89:12: note: remove std::move call here
    return std::move(Instructions);
           ^~~~~~~~~~            ~