[Nios2] Remove Nios2 backend

As mentioned here http://lists.llvm.org/pipermail/llvm-dev/2019-January/129121.html This backend is incomplete and has not been maintained in several months.

Differential Revision: https://reviews.llvm.org/D56691

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351231 91177308-0d34-0410-b5e6-96231b3b80d8
61 files changed